Add excoveralls example

There was no example for Elixir in gitlab-ci examples, this commit adds
the missing example.

excoveralls repository was pointing to an old example that doesn't exist
anymore.

app/views/projects/pipelines_settings/show.html.haml

@@ -86,6 +86,9 @@
               %li
                 tap --coverage-report=text-summary (NodeJS) -
                 %code ^Statements\s*:\s*([^%]+)
+              %li
+                excoveralls (Elixir) -
+                %code \[TOTAL\]\s+(\d+\.\d+)%
 
         = f.submit 'Save changes', class: "btn btn-save"