Bump pygments from 2.1 to 2.7.4

Bumps [pygments](https://github.com/pygments/pygments) from 2.1 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1...2.7.4)

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

Author: dependabot[bot]

requirements.txt

@@ -19,7 +19,7 @@ Flask-WTF==0.14.2
 speaklater==1.3
 Pillow==3.2.0
 misaka==2.0.0
-Pygments==2.1
+Pygments==2.7.4
 pytz==2018.3
 redis==2.10.6
 psycopg2-binary==2.7.5