Do not run the specs on Ruby 2.4

The gitlab gem dropped support for Ruby 2.4: https://github.com/prontolabs/pronto/runs/2041832382?check_suite_focus=true#step:3:33

.github/workflows/specs.yml

@@ -20,6 +20,7 @@ jobs:
           - "3.0"
         exclude:
           - ruby-version: "2.3" # Rugged uses the wrong openssl version on CI and segfaults (similar to https://github.com/libgit2/rugged/issues/718)
+          - ruby-version: "2.4" # The current version of the Gitlab gem dropped support for Ruby 2.4 https://github.com/prontolabs/pronto/runs/2041832382?check_suite_focus=true#step:3:33
     steps:
       - uses: actions/checkout@v2
       - name: Set up Ruby