Eliminate travis-ci cache thrash.

Noticed cache packs like this:
```
store build cache
change detected:
/home/travis/build/pantsbuild/pants/build-support/isort.venv/lib/python2.7/distutils/__init__.pyo
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2html.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2html.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2latex.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2latex.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2man.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2man.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2odt_prepstyles.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2odt_prepstyles.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2odt.pyc
/home/travis/build/pantsbuild/pants/build-support/pants_dev_deps.venv/bin/rst2odt.pyc
/home/travis/bu
...
changes detected, packing new archive
```

Testing Done:
CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/84594779

Bugs closed: 2355

Reviewed at https://rbcommons.com/s/twitter/r/2957/

.travis.yml

@@ -6,6 +6,10 @@ sudo: false
 
 
 before_cache:
+  # Kill all python bytecode in our cached venvs.  Some files appear to
+  # get bytecode compiled in non-yet-understood circumstances leading to
+  # a full cache re-pack due to new bytecode files.
+  - find build-support -name "*.py[co]" -print -delete
   # The `ivydata-*.properties` & root level `*.{properties,xml}` files'
   # effect on resolution time is in the noise, but they are
   # re-timestamped in internal comments and fields on each run and this