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

script/grammar-compiler

@@ -6,6 +6,8 @@ cd "$(dirname "$0")/.."
 image="linguist/grammar-compiler:latest"
 mkdir -p grammars
 
+docker pull $image
+
 exec docker run --rm \
     -u $(id -u $USER):$(id -g $USER) \
     -v $PWD:/src/linguist \