travis: Use gcc-6 for llvm-7 build
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: e7f1ab7c2a259259587475a3e7520e757882f167@356009 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc