Contribution Documentation + Ruby 2.6.4 in Travis

[masonhensley]

Hello @michelson

Quick documentation update for future contributors based on feedback in #46 & #45

• Updated contribution documentation
• Added ruby 2.6.4 to travis pipelines matrix for Rails 6

Thanks for the project 🎉

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Addition reads as follows:

Contributing

• Just fork the repo and send us a Pull Request, with some tests please :)
• FYI: The CI pipeline on travis will require gemlocks for each or the Rails versions we test against.
  • Run bundle exec appraisal install and commit the generated files in the ./gemfile/ directory
  • Check out Appraisal for more context.

[coveralls]

Pull Request Test Coverage Report for Build 163

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 94.193%

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Totals
Change from base Build 154:	0.1%
Covered Lines:	957
Relevant Lines:	1016

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💛 - Coveralls

[michelson]

it seems that I have no access to this repo anymore cc @patriciojofre ??

[masonhensley]

Closing in favor of #48