Upgrade re2 gem to v1.7.0

https://code-review.googlesource.com/c/re2/+/61250 made the re2
library depend directly on abseil. The latest version of abseil now
requires C++14, and the upstream gem has been updated to supply the
C++ compiler flags necessary to build the gem
(https://github.com/mudge/re2/pull/65).

Note that `gem pristine re2` should work now , but if the `--with-re2`
argument is passed in the abseil path won't be found automatically
until https://github.com/mudge/re2/pull/66 is merged.

Changelog: changed