Remove old gemspec dependencies and use only Gemfile

active_support was removed from Rubygems. Bundler was failing.

> $ bundle install
> Your Gemfile lists the gem rspec (>= 0) more than once.
> You should probably keep only one of them.
> While it's not a problem now, it could cause errors if you change the version of just one of them later.
> Fetching gem metadata from https://rubygems.org/.........
> Resolving dependencies...
> Could not find gem 'active_support (>= 3.0) ruby', which is required by gem 'pathfinder_rpg_character (>= 0) ruby', in any of the sources.