Don't preprend v when elixir version does not start with digits (#61)

.github/workflows/ubuntu.yml

@@ -103,6 +103,9 @@ jobs:
           - elixir-version: '1.10.3'
             otp-version: '22.3.4.1'
             os: 'ubuntu-20.04'
+          - elixir-version: 'master'
+            otp-version: '23.1'
+            os:  'ubuntu-20.04'
     steps:
       - uses: actions/checkout@v2
       - name: Use erlef/setup-beam