upgrade travis ci to use jdk 8

Review Request #4127 - Created July 30, 2016 and submitted

Yi Cheng
3735, 3741
benjyw, jsirois, kwlzn, nhoward_tw, patricklaw, stuhood, zundel

Following the suggestion from https://github.com/travis-ci/travis-ci/issues/6371#issuecomment-235755718, use the new image then invoke jdk_switcher for set JAVA_HOME to jdk8.

Changes include:
1. use python 2.7.11 because the built in python 2.7.10 in the trusty image is compiled with 2 byte unicode, and cffi wheels are built with 4 byte unicode. https://github.com/travis-ci/travis-ci/issues/5107#issuecomment-204623681
2. explicitly requires cffi as lmdb conditionally requires it. https://github.com/dw/py-lmdb/blob/master/setup.py#L124-L140
3. add --jvm-platform-default-platform=java6 to some tests for backwards compatibility.


Eric Ayers
Yi Cheng
Eric Ayers
Stu Hood
Stu Hood
Yi Cheng
Review request changed

Status: Closed (submitted)

Change Summary:

95a07b953988fe6604fed192ba2dc61e92183998 thanks!

Eric Ayers

Just a note: adding the cffi dep has made it more difficult for me to make a release and distribute pants. It adds in a binary dependency on libffi and we don't have libffi installed on our CI machines or builder.

  1. See https://rbcommons.com/s/twitter/r/4314/

  2. sorry, wrong RB