Bump matplotlib from 2.2.3 to 2.2.4 (#81)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 2.2.3 to 2.2.4.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](https://github.com/matplotlib/matplotlib/compare/v2.2.3...v2.2.4

)

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requirements.txt

@@ -2,7 +2,7 @@ backports.tempfile==1.0
 futures==3.2.0
 fuzzywuzzy==0.17.0
 lxml==4.3.3
-matplotlib==2.2.3
+matplotlib==2.2.4
 numpy==1.16.2
 Pillow>=3.4.1
 python-crfsuite==0.9.6