Fix for "Deploy to Heroku" button

[katriellucas]

As the title says, removed the deprecated dependecies (mLab addon), added some important (I think) environment variables in the the app creation. Now you directly need to link a database (e.g., MongoDB Atlas) to use Ackee.

Here is an example button:

[coveralls]

Pull Request Test Coverage Report for Build 554

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 46.547%

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Totals
Change from base Build 543:	0.0%
Covered Lines:	448
Relevant Lines:	772

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💛 - Coveralls

[electerious]

Thanks @Go-Merk!

@aleccool213 You're my Heroku specialist 😃 Does this look good to you?

[aleccool213]

Good idea putting this in. I didn't originally put this in because the add-on put in a mongo db uri env var automatically.

[aleccool213]

Is there docs saying which third party mongo dbs are best to use?

[katriellucas]

Honestly, for now I'm using Atlas which is basically mLab (is now part of MongoDB, Inc) making then some sort of official database, it's still the 500mb storage, #149 issue shows some free database hosting companies, Kintohub was the best in my opinion.