Bump httplib2 from 0.12.1 to 0.18.0

Bumps [httplib2](https://github.com/httplib2/httplib2) from 0.12.1 to 0.18.0.
- [Release notes](https://github.com/httplib2/httplib2/releases)
- [Changelog](https://github.com/httplib2/httplib2/blob/master/CHANGELOG)
- [Commits](https://github.com/httplib2/httplib2/compare/v0.12.1...v0.18.0)

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

requirements.txt

@@ -5,7 +5,7 @@ cryptography==2.6.1
 distro==1.4.0
 entrypoints==0.3
 Flask==1.0.2
-httplib2==0.12.1
+httplib2==0.18.0
 itsdangerous==1.1.0
 jeepney==0.4
 Jinja2==2.10.1

requirements_dev.txt

@@ -18,7 +18,7 @@ entrypoints==0.3
 filelock==3.0.10
 flake8==3.5.0
 Flask==1.0.2
-httplib2==0.12.1
+httplib2==0.18.0
 idna==2.8
 imagesize==1.1.0
 itsdangerous==1.1.0