Build bot on travis

[Peter Kuemmel]

Finally I had the time to setup an automatic build for LyX:

    http://syntheticpp.github.io/LyX-bleeding-edge

Runs since several weeks now, so I assume it is quite stable.

It uses MinGW (which is new at all) on Linux.
It generates zips for Windows, master and 2.1.x.
The spell checker stuff is missing due to mingw/3rdparty libs.

At least for Windows users this is a way to get an actual build,
and it could be used by non-hackers to verify a bugfix.

Cheers,
Peter