Support Rails 5.1

[rafaelfranca]

We still have one deprecation that I'm not sure how to solve it.

The deprecation in question was added by @sgrif in https://github.com/rails/rails/commit/4fed08fa787. It seems that code path was deprecated using two premisses:

• define_attribute_method is not common with active record.
• attribute works for all the cases.

Both premisses are invalid with this gem. We do use define_attribute_method and attribute API doesn't work for the specific case of this gem yet.

I tried to override attribute_will_change! in this gem but that would require the attribute_changed? method to know which typed store it is being used.

I'm more inclined to revert that deprecation, since I could not find a way to make the attributes API work for this gem.

Closes #49

[coveralls]

Coverage decreased (-0.2%) to 94.512% when pulling 222bb5b206c186337da559b9c1b53278f9ff24fb on rails-51 into 086557afa82161b6daa100d2231e64ddbf244bad on master.

[coveralls]

Coverage decreased (-0.2%) to 94.512% when pulling 222bb5b206c186337da559b9c1b53278f9ff24fb on rails-51 into 086557afa82161b6daa100d2231e64ddbf244bad on master.

[coveralls]

Coverage decreased (-0.2%) to 94.512% when pulling 222bb5b206c186337da559b9c1b53278f9ff24fb on rails-51 into 086557afa82161b6daa100d2231e64ddbf244bad on master.

[benjaminwood]

Awesome, thanks for this! 🥇