travis: Use gcc-6 for llvm-7 build

llvm does not expose -std=c++11 in cxx flags.
gcc-6 switched default from c++98 to c++14

Reviewers: Aaron Watry, Tom Stellard
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 356009
GitOrigin-RevId: edcf4d28b3b281622c62d138d22c8fd3a0e50b4d
1 file changed