Bump httplib2 from 0.17.2 to 0.18.0 in /gce

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.17.2 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.17.2...v0.18.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

gce/requirements.txt

@@ -5,4 +5,4 @@ Jinja2==2.10.1
 requests==2.20.0
 Werkzeug==0.15.3
 lxml==4.5.0
-httplib2==0.17.2
+httplib2==0.18.0