travis: Switch to Ubuntu 16.04 (xenial)
travis: Switch to Ubuntu 16.04 (xenial)

Use native packages up to llvm-6.

Reviewer: tstellar
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

git-svn-id: e7f1ab7c2a259259587475a3e7520e757882f167@373046 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