Bump httplib2 from 0.13.0 to 0.18.0 (#1633)

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.13.0 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](httplib2/httplib2@v0.13.0...v0.18.0)

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requirements.txt

@@ -13,7 +13,7 @@ elasticsearch-dsl==6.3.1
 geoip2==2.5.0
 GitPython==2.1.3
 hjson==2.0.2
-httplib2==0.13.0
+httplib2==0.18.0
 ipaddr==2.2.0
 ipaddress==1.0.17
 ipwhois==0.15.0