CI: Upgrade geckodriver-helper

[xtrasimplicity]

Summary

Following on from #370, @radar's improvements to geckodriver-helper were merged upstream and released as version 0.20.1, so I've upgraded the appraisals to match ~> 0.20.2 (the newest release as of today) rather than ~> 0.0.5.

Details

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

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This change is 

[olleolleolle]

CI is green!

[xtrasimplicity]

@olleolleolle Just waiting for Travis to finish. :) There's a 90%+ chance it'll be fine, given it's pretty much all passing on Ruby 2.2.9, but I'd rather wait. I can see that the codeship build has finished, but I don't seem to have access to view the results. :/

[olleolleolle]

I had some "banner vision" re: only 1 of the badges is actually Travis.

I looked at the wrong thing, and the CI is not yet visibly green

[xtrasimplicity]

All good! 😀

[luke-hill]

For ref, webdrivers will manage all path requirements (Not just gecko), if you wanted something more robust. Also exporting the path is possible in travis and would be easier than using a gem?

[xtrasimplicity]

@luke-hill Thanks so much for the suggestion - it sounds great! I'll check it out when I get a moment. 👍