Travis CI for glslang broken by latest googletest

[greg-lunarg]

The current googletest top-of-tree is breaking Travis CI for glslang. See #2472

The googletest problem is documented by issue google/googletest#3128

The googletest problem was introduced by google/googletest@fbef071

There are a number of ways to fix this problem.

1. hack .travis.yml to checkout a known good version of googletest after cloning it

2. make googletest part of the glslang known-good.json mechanism. These versions are fetched by update_glslang_sources.py, which is currently called by .travis.yml.

3. Another is some solution involving submodules. This would be a longer term solution.

#1 is probably the easiest and fastest solution. That is my vote. I will submit a PR shortly if I don't hear any concerns.