twitter.commons: Release 0.6.0 of twitter.common.pants

Release twitter.common.pants 0.6.0 due to changed transitive deps.

Testing Done:
A functional noop; a test release was incorporated internally to unblock a release.

Reviewed at https://rbcommons.com/s/twitter/r/1329/
Fixing VarsJsonHandler and VarsHandlerTest for Java 7+ which changes hashmap iteration order

Testing Done:
Ran all java tests in commons with:

./pants goal test {src,tests}/java/com/twitter/common::
./pants goal test {src,tests}/scala/com/twitter/common::

under Java 7 and re-ran the java tests with Java 8

Reviewed at https://rbcommons.com/s/twitter/r/1368/