Allow httpclient 2.x

Follow up to #57. Sorry for making trouble :bow: 

> Do you know if httpclient follows semantic versioning ? Maybe the selector should be widened to ~2.0 so existing users aren't forced to upgrade. There's a common issue where gem A depends on 2.3.0 and B on 2.4.0 and the user of both is blocked.

As I see, httpclient seems to be following semvar(https://github.com/nahi/httpclient#changes).
And also, author of httpclient is an open source veteran, so I think he won't introduce breaking changes on minor update.
I couldn't find `pusher-gem` works well with `httpclient` between `=> 2.0` and `< 2.3`, so I specified `~> 2.3`.