Rails edge (5) and Formtastic 2.3.x fixes

2.3-stable is failing build on edge Rails, so I wanted to see what it would take to add support. Here's what I found it took to get [green](https://travis-ci.org/justinfrench/formtastic/builds/34484370) for 2.3-stable for the edge Rails builds (which we allow to fail in a regular green build):

- [x] Different nokogiri versions for Ruby < 1.9.3 — Rails 5 requires nokogiri >= 1.6.0, but this requires Ruby 1.9.2 and Formtastic 2.3 still supports Ruby 1.8.7, so were testing the Ruby version in `formtastic.gemspec`
- [x] Some mock and gem fixes already resolved in 1aed21b needed to be copied over
- [x] ammeter 1.1.1 wasn't compatible with Ruby 1.8.7, but they brought it back in 1.1.2

While I was here I added missing support for Rails 4.1 in the build matrix.