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>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@356009 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed