Add (CPU-only) test, lint, warn and parallel (-j 8) 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