Update .travis.yml

You should use the dedicated option of our CMakeLists.txt as the ```BUILD_SHARED_LIBS``` variable will get overwritten within the first lines (see https://github.com/hpicgs/glbinding/blob/master/CMakeLists.txt#L39). Either way, building glbinding as shared library is the default setting.
Merge pull request #3 from scheibel/patch-1

Update .travis.yml