Add Clang 5.0 to the Travis CI matrix.
Add Clang 5.0 to the Travis CI matrix.

Change-Id: Idf299f63dfc7b95457158d2eb0a35598a3d0c8f4
Reviewed-on: https://code-review.googlesource.com/16810
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2