Add head for ruby to workflow. (#836)

- removes checks for grape 1.3.3

Author: LeFnord

.github/workflows/ruby.yml

@@ -13,8 +13,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0']
-        grape-version: [1.5.3, 1.4.0, 1.3.3]
+        ruby-version: ['2.6', '2.7', '3.0', 'head']
+        grape-version: [1.5.3, 1.4.0]
         model-parser: [grape-swagger-entity, grape-swagger-representable, '']
 
     steps:
@@ -28,5 +28,3 @@ jobs:
         bundler-cache: true
     - name: Run rspec
       run: bundle exec rspec
-    - name: Run rubocop
-      run: bundle exec rubocop