Add (CPU-only) test, lint, warn and parallel (-j 4) to travis CI build.
Add /usr/local/lib to travis config (seems needed for LMDB).
Allow users to override the default compiler by specifying a CXX in
their Makefile.config.  (The CUSTOM_CXX still works fine.)
Create runtestnogpu Makefile target; use in .travis.yml.
Put export in "before_script"; disable clang build -- doesn't work on Linux.
Cache Ubuntu apt packages.
Install bc package to hopefully suppress "bc: not found" errors
Get apt packages before_install as suggested by Travis official docs
Travis build failure wasn't working for lint/warn because they didn't
exit with a non-zero code -- this fixes that.
Remove specified email address and IRC notifications (emails are sent
to the committer by default; others can view build results in public
Travis feed, on pull requests, etc.).