Fix paths for Docker on Windows

When using on Windows with the default docker configuration, it passes `.serverless\\requirements.txt` to `docker ... pip install ...`. This results in the following error:

```
Could not open requirements file: [Errno 22] Invalid argument: '.serverless\\requirements.txt'
```

To fix this, we can convert the paths from Windows style to Linux style when running on Windows with `dockerZip: true`.

Author: kichik

lib/pip.js

@@ -40,7 +40,7 @@ function installRequirements(requirementsPath, targetFolder, serverless, service
   let cmdOptions;
   let pipCmd = [
     options.pythonBin, '-m', 'pip', '--isolated', 'install',
-    '-t', targetRequirementsFolder, '-r', fileName,
+    '-t', dockerPathForWin(options, targetRequirementsFolder), '-r', dockerPathForWin(options, fileName),
     ...options.pipCmdExtraArgs,
   ];
   if (!options.dockerizePip) {
@@ -122,6 +122,13 @@ function installRequirements(requirementsPath, targetFolder, serverless, service
   }
 };
 
+function dockerPathForWin(options, path) {
+  if (process.platform === 'win32' && options.dockerizePip) {
+    return path.replace('\\', '/');
+  }
+  return path;
+}
+
 /**
  * pip install the requirements to the .serverless/requirements directory
  * @return {undefined}