Fix and expand upon our Travis CI configuration.

 * CHANGE .travis.yml so that commands for different purposes (e.g. getting
   dependencies, building, testing) are in separate config lines and sections.
 * CHANGE .travis.yml to use their mechanism for installing dependencies via
   apt. [0]  This also allows us to not need sudo (the "sudo: false" line).
 * CHANGE Travis CI tests (the "script:" section) to build and run tests in the
   same manner as Jenkins (i.e. with --enable-fatal-warnings and
   --disable-silent-rules and run `make check`).
 * ADD Travis configuration to do all the target builds with both GCC and clang.
 * ADD make flags to build with both of the cores available.
 * ADD notifications for IRC, and configure email notifications (to the author
   of the commit) only if the branch was previously building successfully and
   the latest commit broke it.
 * ADD the ability to run the Travis build matrix for OSX as well, but leave it
   commented out by default (because it takes roughly ten times longer, due to a
   shortage of OSX build machines).
 * ADD Travis config option to cancel/fail the build early if one target has
   already failed ("fast_finish: true").
 * ADD comments to describe what our Travis config is doing and why it is
   configured that way.

[0]: https://docs.travis-ci.com/user/installing-dependencies/#Installing-Packages-on-Container-Based-Infrastructure)