Remove a C99ism from dispatch.

I've decided it's time to accept that building with MSVC 2010 is a way
that people want to use this software.
Add a control file for Travis CI.

This isn't quite as good as I'd like -- the XVFB setup doesn't support
a bunch of context creation extensions that we use, and I haven't
looked into the multiplatform stuff for doing OS X testing yet.  I
think it's a good start, though.