Fix a typo in .travis.yml

The old code was trying to pass -Wgnu-zero-variadic-macro-arguments
as part of CXXFLAGS, but it forgot the quotation marks needed around
whitespace. This meant that option was ignored:
https://travis-ci.org/github/google/googletest/jobs/666534177#L760

Unfortunately, the codebase is not remotely clean with respect to that
warning option. It fails like this:
https://travis-ci.org/github/Quuxplusone/googletest/jobs/668118135

So, remove that failing configuration from the test matrix until
someone has time to look at it.

.travis.yml

@@ -26,9 +26,6 @@ matrix:
     - os: linux
       compiler: gcc
       env: BUILD_TYPE=Debug VERBOSE=1 CXX_FLAGS=-std=c++11
-    - os: linux
-      compiler: clang
-      env: BUILD_TYPE=Release VERBOSE=1 CXX_FLAGS=-std=c++11 -Wgnu-zero-variadic-macro-arguments
     - os: linux
       compiler: clang
       env: BUILD_TYPE=Release VERBOSE=1 CXX_FLAGS=-std=c++11 NO_EXCEPTION=ON NO_RTTI=ON COMPILER_IS_GNUCXX=ON