Use a test param for the CPU/GPU switch; the macro probably isn't
kosher.  Create runtestnogpu target.
@huyng's .travis.yml integration configuration file to install and test Caffe.
Allow users to override the default compiler by specifying a CXX in
their Makefile.config, as Travis exports CXX as the compiler env variable
name. (The CUSTOM_CXX still works fine.)
@jeffdonahue's tweaks to .travis.yml config:
-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).
-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
-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.).
Travis build failure wasn't working for lint/warn because they didn't
exit with a non-zero code -- this fixes that.