Merge remote-tracking branch 'upstream/develop' into develop

* upstream/develop:
Merge branch 'develop' of github.com:twitter/scalding into develop

* 'develop' of github.com:twitter/scalding:
  "Require" sources in JobTests to have a mock buffer.
  Don't use a bad 2.10 version
  Fix typo
  Re-add mapValues to IdentityReduce
  make sure to push filterKeys mapside
  add filterKeys
  Add .filter to KeyedListLike
Make sure to set cascading.app.appjar.class so that cascading can find job classes on mappers/reducers
Merge pull request #762 from themodernlife/develop

Issues using skinny jars, -libjars flag and com.twitter.scalding.Tool as the Main class