Adds httpoison ~> 2.0 support

[antedeguemon]

Hey there! 🖖

This PR simply loosens the httpoison ~> 1.0 constraint.

The breaking changes from httpoison 1.8.2 to 2.0 are related to SSL options - which are not used by exvcr. Therefore, loosening the constraint shouldn't have any collateral effect on exvcr.

For more details about the breaking changes, see httpoison 2.0.0 release.

Closes #195

[coveralls]

Coverage: 88.784%. Remained the same when pulling fc2ea299ec4aad474ddea20d0c26a77f8a75616b on antedeguemon:vm-adds-httpoison-2.0-support into 2204e8f72b103d2fade92cd0b043fb4f17872618 on parroty:master.

[coveralls]

Coverage: 90.363% (+1.6%) from 88.784% when pulling fc2ea299ec4aad474ddea20d0c26a77f8a75616b on antedeguemon:vm-adds-httpoison-2.0-support into 2204e8f72b103d2fade92cd0b043fb4f17872618 on parroty:master.

[parroty]

Thanks!

[nathany-copia]

Thanks!