Change path-to-regexp references to ^1.7.0 (#6117)

The most recent `path-to-regexp` versions are backward incompatible:
https://github.com/pillarjs/path-to-regexp#compatibility-with-express--4x
https://github.com/pillarjs/path-to-regexp/blob/master/History.md#200--2017-08-23

Author: dosentmatter

packages/react-router/docs/api/Redirect.md

@@ -16,7 +16,7 @@ import { Route, Redirect } from 'react-router'
 
 ## to: string
 
-The URL to redirect to. Any valid URL path that [`path-to-regexp`](https://www.npmjs.com/package/path-to-regexp) understands.
+The URL to redirect to. Any valid URL path that [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0) understands.
 All URL parameters that are used in `to` must be covered by `from`.
 
 ```jsx
@@ -25,7 +25,7 @@ All URL parameters that are used in `to` must be covered by `from`.
 
 ## to: object
 
-A location to redirect to. `pathname` can be any valid URL path that [`path-to-regexp`](https://www.npmjs.com/package/path-to-regexp) understands.
+A location to redirect to. `pathname` can be any valid URL path that [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0) understands.
 
 ```jsx
 <Redirect to={{
@@ -47,8 +47,8 @@ When `true`, redirecting will push a new entry onto the history instead of repla
 
 ## from: string
 
-A pathname to redirect from. Any valid URL path that [`path-to-regexp`](https://www.npmjs.com/package/path-to-regexp) understands.
-All matched URL parameters are provided to the pattern in `to`. Must contain all parameters that are used in `to`. Additional parameters not used by `to` are ignored. 
+A pathname to redirect from. Any valid URL path that [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0) understands.
+All matched URL parameters are provided to the pattern in `to`. Must contain all parameters that are used in `to`. Additional parameters not used by `to` are ignored.
 
 This can only be used to match a location when rendering a `<Redirect>` inside of a `<Switch>`. See [`<Switch children>`](./Switch.md#children-node) for more details.
 

packages/react-router/docs/api/Route.md

@@ -132,7 +132,7 @@ This could also be useful for animations:
 
 ## path: string
 
-Any valid URL path that [`path-to-regexp`](https://www.npmjs.com/package/path-to-regexp) understands.
+Any valid URL path that [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0) understands.
 
 ```jsx
 <Route path="/users/:id" component={User}/>

packages/react-router/docs/guides/migrating.md

@@ -198,7 +198,7 @@ In v4, you must re-pass these properties to the `to` prop:
 ## PatternUtils
 
 ### matchPattern(pattern, pathname)
-In v3, you could use the same matching code used internally to check if a path matched a pattern. In v4 this has been replaced by [matchPath](/packages/react-router/docs/api/matchPath.md) which is powered by the [path-to-regexp](https://github.com/pillarjs/path-to-regexp) library.
+In v3, you could use the same matching code used internally to check if a path matched a pattern. In v4 this has been replaced by [matchPath](/packages/react-router/docs/api/matchPath.md) which is powered by the [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0) library.
 
 ### formatPattern(pattern, params)
 In v3, you could use PatternUtils.formatPattern to generate a valid path from a path pattern (perhaps in a constant or in your central routing config) and an object containing the names parameters:
@@ -210,7 +210,7 @@ const THING_PATH = '/thing/:id';
 <Link to={PatternUtils.formatPattern(THING_PATH, {id: 1})}>A thing</Link>
 ```
 
-In v4, you can achieve the same functionality using the [compile](https://github.com/pillarjs/path-to-regexp#compile-reverse-path-to-regexp) function in [path-to-regexp](https://github.com/pillarjs/path-to-regexp).
+In v4, you can achieve the same functionality using the [compile](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0#compile-reverse-path-to-regexp) function in [`path-to-regexp@^1.7.0`](https://github.com/pillarjs/path-to-regexp/tree/v1.7.0).
 
 ```jsx
 // v4