Upgrade mime-types-data to 3.2019.0331

This saves some disk space by matching the version used in Gitaly.

Only minor changes:
https://github.com/mime-types/mime-types-data/blob/master/History.md

Gemfile.lock

@@ -499,7 +499,7 @@ GEM
     method_source (0.9.2)
     mime-types (3.2.2)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2018.0812)
+    mime-types-data (3.2019.0331)
     mimemagic (0.3.2)
     mini_magick (4.8.0)
     mini_mime (1.0.1)