release 2018-07-01

Include operator<<() for StringPiece in libre2.so

operator<<() for StringPiece was moved under namespace re2 in an
earlier refactoring, but the exported symbols weren't updated with
the new mangled name.

Change-Id: If113a21d1ee136d06055e63ff89e486b73e07407
Reviewed-on: https://code-review.googlesource.com/29410
Reviewed-by: Paul Wankadia <junyer@google.com>