Add minimal OrderedHash and remove runtime activesupport dependency.

[omghax]

As discussed in #94, this PR adds a basic OrderedHash and makes activesupport a development dependency rather than a runtime one.

[nbibler]

It seems to be good to get away from activesupport features in production (thus the recreation of OrderedHash). For example, if an Appraisal configuration for a project uses a version of activesupport that is either missing OrderedHash or has a broken implementation of OrderedHash it would change the functionality of Appraisal. In that case, it would be non-obvious to diagnose and likely produce surprising side effects during execution.

Using OrderedHash here seems like it's a bit of overkill for the current usage, but this patch is certainly a short distance to removing the immediate need for a production dependency on activesupport. 👍

[sikachu]

I merged this as 108d4c5.