Release v6.0.0 (#4002)

* Update all submodules

* Ensure always using lastest docker image

* Allow passing in GEM_VERSION from env

This is useful to building test gems in a cache friendly way using:
`GEM_VERSION=$(git describe --tags 2>/dev/null | sed 's/-/./g' | sed
's/v//') bundle exec rake build_gem`

* Update submodules one last time

* Set version 6.0.0

lib/linguist/version.rb

@@ -1,3 +1,3 @@
 module Linguist
-  VERSION = "5.3.3"
+  VERSION = "6.0.0"
 end