Make webvtt-config.h depend on Makefile.

This way it gets remade if the generating rule might have changed.
Modified .travis.yml to test 'make distcheck' as well. In addition, also trying to send irc notifications to #seneca on irc.freenode.net now.
Updated test/unit/Makefile.am to list subdirectories containing test files in EXTRA_DIST
Append to EXTRA_DIST instead of clobbering it.

The gtest fragment EXTRA_DIST line was shadowing the earlier
one for the test file subdirectories. Bad merge.