Bump httplib2 from 0.9.2 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.9.2 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@0.9.2...v0.18.0)

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

Author: dependabot[bot]

requirements.txt

@@ -11,7 +11,7 @@ ecdsa==0.13
 enum==0.4.6
 graypy==0.2.14
 gunicorn==19.9.0
-httplib2==0.9.2
+httplib2==0.18.0
 ipaddress==1.0.16
 ipaddr==2.1.11
 ipython==5.3.0

requirements_docs.txt

@@ -15,7 +15,7 @@ djangorestframework==2.4.3
 ecdsa==0.13
 enum==0.4.6
 gunicorn==19.9.0
-httplib2==0.9.2
+httplib2==0.18.0
 ipaddress==1.0.16
 ipython==5.3.0
 json-delta==2.0