update rules_scala version to latest in readme (#237)

Author: ittaiz

README.md

@@ -23,7 +23,7 @@ you must have bazel 0.3.1 or later and add the following to your WORKSPACE file:
 
 ```python
 
-rules_scala_version="d916599d38de29085e5ca9eae167716c4f150a02" # update this as needed
+rules_scala_version="031e73c02e0d8bfcd06c6e4086cdfc7f3a3061a8" # update this as needed
 
 http_archive(
              name = "io_bazel_rules_scala",