Add GCC 9.x to the Travis CI matrix.
Add GCC 9.x to the Travis CI matrix.

Change-Id: I5f7e3ea5fdd23ff181f8d1037e86f7578d21ab0b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/41350
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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