Updated Rails dependency

[EtienneDepaulis]

Hello guys, I'm currently playing with your GDPR engine. Thanks a lot for your work.

I've created a Rails sample app (https://github.com/EtienneDepaulis/rails_gdpr_sample_app) to try it more seriously before implementing on our production app.
I've noticed that you have pretty strict Rails requirements, every things seems to be working fine with 5.2.

In this PR I've also added common gitignore files and removed the Gemfile.lock (it looks like it is a best practice on other rails engines repo).

One question though : what about removing the mysql dependency for local work on the gem ?

[coveralls]

Pull Request Test Coverage Report for Build 25

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 93.511%

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Files with Coverage Reduction	New Missed Lines	%
test/dummy/app/models/user.rb	3	55.56%

Totals
Change from base Build 23:	-0.3%
Covered Lines:	735
Relevant Lines:	786

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💛 - Coveralls

[michelson]

Hi @EtienneDepaulis , currently at prey we use rails 5.1 so we haven't tested yet on rails 5.2. so is great news that is working on latest versions of rails. Maybe it 's a good idea to use appraisals gem to test different versions. This PR is fine for me so I will merge this. Thanks for this addition !