CI fails for Firefox

[jelhan]

CI configuration does not seem to be correct for Firefox. Seeing a repository migrated from TravisCI with CI configured to run both on Chrome and Firefox fail with:

 not ok 1 Firefox - [undefined ms] - error
    ---
        message: >
            Error: Browser exited unexpectedly
            Non-zero exit code: 1
            Stderr: 
             Error: no DISPLAY environment variable specified
            
            
        browser log: |
            [object Object]
            [object Object]
            [object Object]
    ...

See this job as an example.

[jelhan]

Repository was not using Firefox before with TravisCI. It was misleading due to #10. That bug caused default values to be used instead of existing TravisCI configuration. The default values include Chrome and Firefox as browsers.