limit support to Rails >= 3.2 and Ruby >= 1.9

#### Removed outdated version-checking conditions
Modules affected:
+ CSV parser
+ asset registration
+ class reloading in development

#### Removed 99% of our custom class reloader in favor of Rails reloading
Note: I removed the code that purges `app/admin` from Rails' `eager_load_paths`
since there were no bad effects of doing so. Only the code that does this for
Rails' `autoload_paths` seems necessary.

#### Gemspec dependencies updated
+ fastercsv removed
+ coffee-rails added
+ sass replaced by sass-rails

# Please enter the commit message for your changes. Lines starting
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# An empty message aborts the commit.
# On branch metasearch->ransack
# Changes to be committed:
#   (use "git reset HEAD^1 <file>..." to unstage)
#
#	modified:   .travis.yml
#	modified:   Gemfile
#	modified:   activeadmin.gemspec
#	modified:   app/views/active_admin/resource/index.csv.erb
#	modified:   features/development_reloading.feature
#	modified:   features/step_definitions/format_steps.rb
#	modified:   lib/active_admin.rb
#	modified:   lib/active_admin/application.rb
#	deleted:    lib/active_admin/dependency_checker.rb
#	modified:   lib/active_admin/engine.rb
#	modified:   lib/active_admin/filters/resource_extension.rb
#	modified:   lib/active_admin/inputs/filter_select_input.rb
#	deleted:    lib/active_admin/reloader.rb
#	deleted:    lib/active_admin/sass/active_admin.scss
#	deleted:    lib/active_admin/sass/css_loader.rb
#	deleted:    lib/active_admin/sass/helpers.rb
#	modified:   spec/support/detect_rails_version.rb
#	modified:   spec/unit/application_spec.rb
#	modified:   spec/unit/rails_spec.rb
#	deleted:    spec/unit/reloader_spec.rb
#	deleted:    spec/unit/sass/helpers_spec.rb
#