Bump mime-types to 3.2.2

Full list of changes:
https://github.com/mime-types/ruby-mime-types/blob/master/History.md

This matches the version shipped in gitaly-ruby.

Gemfile.lock

@@ -449,9 +449,9 @@ GEM
     memoizable (0.4.2)
       thread_safe (~> 0.3, >= 0.3.1)
     method_source (0.9.0)
-    mime-types (3.1)
+    mime-types (3.2.2)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2016.0521)
+    mime-types-data (3.2018.0812)
     mimemagic (0.3.0)
     mini_magick (4.8.0)
     mini_mime (1.0.1)