Fix power_enum to work with Rails 4.2

Looks like Rails 4.2.0 removed the first `macro` argument to
`ActiveRecord::Reflection::MacroReflection#initialize` - see
rails/rails@f8d2899
Fix enumeration_reflection_spec

Fix enumeration reflection spec to not test macro, since it's not
present anymore
Revert "Fix enumeration_reflection_spec"

This reverts commit c8e587213dbc703a9cce6efd7bd0ea54d37fe0c5.
Initialize conditionally based on rails version
Merge branch 'feature/rails-4.2' of git://github.com/glittershark/power_enum_2 into glittershark-feature/rails-4.2
Fixing support for Rails 4.2 so it actually works.
Merge branch 'glittershark-feature/rails-4.2'

* glittershark-feature/rails-4.2:
  Fixing support for Rails 4.2 so it actually works.
  Initialize conditionally based on rails version
  Revert "Fix enumeration_reflection_spec"
  Fix enumeration_reflection_spec
  Fix power_enum to work with Rails 4.2