Fixup a hole in external dependency listing wrt encoding.

Refer to https://rbcommons.com/s/twitter/r/615/
This fixes a hole left there encountered at Twitter against ... shapeless.
It also adds an integration test that is now fixed with this fix.

Testing Done:
Locally:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test:all
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/tasks:jvm_bundle_integration

Travis CI went green: https://travis-ci.org/pantsbuild/pants/builds/31093736

Bugs closed: 406

Reviewed at https://rbcommons.com/s/twitter/r/776/