Fixed method rails_loaded? for when Rails is defined but no application is actually loaded

This is like #127 but only a tiny patch and fixes the issues I'm having with:

```
NoMethodError on line ["221"] of /Users/parndt/.gem/ruby/2.1.1/gems/compass-rails-1.1.3/lib/compass-rails.rb: undefined method `config' for nil:NilClass
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-rails-1.1.3/lib/compass-rails.rb:128:in `configuration'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/configuration/helpers.rb:44:in `configuration_for'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/configuration/helpers.rb:22:in `add_configuration'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/configuration/helpers.rb:107:in `add_project_configuration'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/commands/project_base.rb:31:in `add_project_configuration'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/commands/project_base.rb:25:in `configure!'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/commands/project_base.rb:15:in `initialize'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/commands/update_project.rb:37:in `initialize'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/exec/sub_command_ui.rb:42:in `new'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/exec/sub_command_ui.rb:42:in `perform!'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/lib/compass/exec/sub_command_ui.rb:15:in `run!'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/bin/compass:30:in `block in <top (required)>'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/bin/compass:44:in `call'
  /Users/parndt/.gem/ruby/2.1.1/gems/compass-0.12.2/bin/compass:44:in `<top (required)>'
  /Users/parndt/.gem/ruby/2.1.1/bin/compass:23:in `load'
  /Users/parndt/.gem/ruby/2.1.1/bin/compass:23:in `<main>'

```