Use Poison atom keys

[jwarlander]

Instead of manually converting JSON keys to atoms in Dayron.HTTPoisonAdapter, eliminate a dependency and some code by using the keys: :atoms option for Poison when decoding.

[coveralls]

Coverage remained the same at 100.0% when pulling 076cdcc on jwarlander:use-poison-atom-keys into 6d06d94 on inaka:master.

[cloud8421]

Great to drop a dependency, but this triggered a question around dynamic atoms for me, I'll raise a n issue.

[jwarlander]

@cloud8421, it should behave the same as the code it replaces (https://github.com/devinus/poison/blob/master/lib/poison/parser.ex#L113-L114) - eg. yes, it will create potentially infinite atoms until the VM crashes.

As mentioned in #57, this can't really be changed until there's a new major release, as it would potentially break existing use cases, so I just went for eliminating a dependency for now.

[cloud8421]

@jwarlander thanks, I raised a separate issue at #59 about it. Yes, it definitely needs a deprecation/upgrade path if it gets changed.