Bump pillow from 2.3.0 to 6.2.0

Bumps [pillow](https://github.com/python-pillow/Pillow) from 2.3.0 to 6.2.0.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](python-pillow/Pillow@2.3.0...6.2.0)

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

Author: dependabot[bot]

requirements.txt

@@ -13,7 +13,7 @@ Mako==0.9.1
 Markdown==2.4
 MarkupSafe==0.23
 PAM==0.4.2
-Pillow==2.3.0
+Pillow==6.2.0
 Pygments==1.6
 Routes==2.0
 SQLAlchemy==1.0.9