Support for Rails 5.1

[lephyrius]

I get this error when doing the bundle:

Bundler could not find compatible versions for gem "activerecord":
  In Gemfile:
    activerecord (~> 5.1.0)

    periscope-activerecord (~> 2.1.1) was resolved to 2.1.1, which depends on
      activerecord (< 5.1, >= 3)

[dja]

@laserlemon Is this an easy fix?

[laserlemon]

Thank you and sorry for the slow response. I wouldn't call it easy but it's in progress at #14. Please take a look! As soon as the Travis build agrees that everything is 💚, I'll merge and cut new releases of periscope-activerecord and periscope-mongoid. Thank you! 👏