[readme] Update recommended toolchain to 'latest'

Now that we are building with HEAD swiftpm, change the recommended
toolchain to be whatever the latest available snapshot is. If you are
developing on ToT sourcekit-lsp, then using the latest snapshot is going
to have the closest match other than building the whole toolchain from
source.

Author: benlangmuir

README.md

@@ -7,7 +7,7 @@ SourceKit-LSP is an implementation of the [Language Server Protocol](https://mic
 
 SourceKit-LSP is under heavy development! The best way to try it out is to build it from source. You will also need a Swift development toolchain and an editor that supports LSP.
 
-1. Install the `swift-DEVELOPMENT-SNAPSHOT-2019-02-14-a` toolchain snapshot from https://swift.org/download/#snapshots. Set the environment variable `SOURCEKIT_TOOLCHAIN_PATH` to the absolute path to the toolchain or otherwise configure your editor to use this toolchain. See [Toolchains](#toolchains) for more information.
+1. Install the latest master toolchain snapshot from https://swift.org/download/#snapshots. Set the environment variable `SOURCEKIT_TOOLCHAIN_PATH` to the absolute path to the toolchain or otherwise configure your editor to use this toolchain. See [Toolchains](#toolchains) for more information.
 
 2. Build the language server executable `sourcekit-lsp` using `swift build`. See [Building](#building-sourcekit-lsp) for more information.
 
@@ -73,7 +73,7 @@ SourceKit-LSP depends on tools such as `sourcekitd` and `clangd`, which it loads
 
 ### Recommended Toolchain
 
-Use the `swift-DEVELOPMENT-SNAPSHOT-2019-02-14-a` toolchain snapshot from https://swift.org/download/#snapshots. SourceKit-LSP is still early in its development and we are actively adding functionality to the toolchain to support it.
+Use the latest master toolchain snapshot from https://swift.org/download/#snapshots. SourceKit-LSP is still early in its development and we are actively adding functionality to the toolchain to support it.
 
 ### Selecting the Toolchain