Include the right thing
Require BigDecimal

This should help with the broken build.

https://travis-ci.org/orgsync/active_interaction/jobs/25522400
Fix Model filter integration test

I'm not sure why it was broken with ActiveModel ~> 3.0. This fixes it,
though.
Use Module.const_get instead of String#constantize

This fixes an incompatibility between major versions of Rails.

    # Rails 3.2
    >> ''.constantize
    => Object

    # Rails 4.1
    >> ''.constantize
    NameError: wrong constant name
Fix version in comment
Fix Error#initialize_dup for Rails < 3.2.13
Require activemodel >= 3.2
Fix RSpec warnings
Fix I18n warning in the specs
Update Vagrant and VirtualBox version hints
Check for responsiveness before setting flag

This is necessary only for Rails 3.2.13.
Merge branch 'master' into explicit-require

Conflicts:
	lib/active_interaction/filters/decimal_filter.rb
Explicitly require ActiveInteraction::Errors

This prevents a superclass mismatch error.