Clean webvtt-config.h.

According to the examples in the automake manual, BUILD_SOURCES
resolves the dependency generation race, but doesn't actualy add
the built sources to the clean file list. We need to do this
explicitly to make sure it's removed from the tree on 'clean'.

This commit has 'make distcheck' working for me for the first time.
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.
Merge pull request #4 from rillian/dev-fixes

More 'distcheck' fixes