Fix Travis-CI build by getting glew from upstream
Fix Travis-CI build by getting glew from upstream

glew 1.6 does not yet declare OpenGL 4's glTexStorage2D, so get glew 1.10 from trusty

szellmann/visionaray,tu500/visionaray,ukoeln-vis/ctpperf,ukoeln-vis/ctpperf,tu500/visionaray,szellmann/visionaray