Revert "Enable bundler cache"

This reverts commit e185045.

It broke Github Actions tests, maybe try again after switching to
CodeCov action.

.github/workflows/main.yml

@@ -21,6 +21,6 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-          bundler-cache: true
+          bundler-cache: false
       - run: bundle install
       - run: bundle exec rspec