Update .travis.yml
Update .travis.yml

Restore clang build to CI, and perform unit test only on GCC build
Smozeley/openMVG,mojovski/openMVG,yoshidaken/openMVG,openMVG/openMVG,openMVG/openMVG,caymard/openMVG,poparteu/openMVG,caymard/openMVG,poparteu/openMVG,Vinzza/openMVG,danylaksono/openMVG,danylaksono/openMVG,yoshidaken/openMVG,mojovski/openMVG,mojovski/openMVG,openMVG/openMVG,Smozeley/openMVG,Vinzza/openMVG,danylaksono/openMVG,Smozeley/openMVG,caymard/openMVG,yoshidaken/openMVG,poparteu/openMVG,openMVG/openMVG,Vinzza/openMVG,poparteu/openMVG,openMVG/openMVG