Add --keep-going flag to first make so that any targets that can be
built are built; write out full -j flag as --jobs for clarity.
Run Travis build on all branches (remove whitelist from .travis.yml).
Replace CUSTOM_CXX env var to specify non-default C++ compiler.
Add blank lines between fields for readability.
Merge pull request #694 from jeffdonahue/travis-tweaks

Travis tweaks -- make --keep-going; build on all branches; replace CUSTOM_CXX <eom>