Bump httplib2 from 0.9.1 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.9.1 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@0.9.1...v0.18.0)

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

Author: dependabot[bot]

requirements.txt

@@ -22,7 +22,7 @@ feedparser==5.2.1
 flake8==3.5.0
 gunicorn==19.6.0
 html5lib==0.9999999
-httplib2==0.9.1
+httplib2==0.18.0
 idna==2.6
 lxml==3.6.1
 mccabe==0.6.1