Remove deprecated RAILS_* constants
Updated README
Upgraded spec_helper for new plugins location
Upgrade plugin to rspec 2.x with bundler
Updated specs for ActiveModel new syntax about errors

See: http://stackoverflow.com/questions/7526499/undefined-method-on-for-actionmodel
Added constraint about Redmine version because we're about to make non 1.x compatible changes
Added some routes since the catch-all route doesn't exist in the core anymore
Updated customers/list icon
Fixed fixtures path in spec_helper
Fixed wrong number of arguments in customers/new partial
Less restrictive routes so that create/update/destroy actions work
Display form correctly in customers/select partial
Fixed bad helper usage in customers/edit partial
Explicitly require users fixtures in controller spec
Updated plugin to work with Redmine 2.6.x