HOTFIX: Remove generated-mima-excludes file after runing MIMA.

This has been causing some false failures on PR's that don't merge
correctly.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #971 from pwendell/mima and squashes the following commits:

1dc80aa [Patrick Wendell] HOTFIX: Remove generated-mima-excludes file after runing MIMA.

dev/mima

@@ -31,4 +31,5 @@ if [ $ret_val != 0 ]; then
   echo "NOTE: Exceptions to binary compatibility can be added in project/MimaExcludes.scala"
 fi
 
+rm -f .generated-mima-excludes
 exit $ret_val