Marked expected failure in GafferImageTest.ImageWriterTest.
Added unit test execution to travis setup.

Not running UI tests at this stage - those can wait until we have the basics working.
Fixed installation to paths starting with "./".
Added skipping of OpenGL tests in travis.

I don't know if it's possible to get OpenGL running in travis yet.
Merge pull request #992 from johnhaddon/travisTesting

Travis testing