Handle BMP with negative height, implement Travis-CI

Hello, I am happily using fastimage to determine image dimensions and happened to discover a bitmap with a negative height, which is apparently allowed but wasn't working properly with fastimage. The first commit here should accommodate negative heights. I added a truncated copy of the image in question to the tests as well.

The next three commits implement Travis-CI for the gem. If you would prefer to not do that, or if you would like those as a separate request, I can handle that.

Thanks for fastimage!