Update gemspec and rake task

Gemspec:
 * Remove railties dependency

Rake task:
 * Support Rake 10 syntax for tasks_in_scope
 * Incorporate asset cleanup task in rake install task
 * Simplify directory handling; update resource paths for Ext 4.2
 * Remove -debug from from generated .css filenames
Update SassExtensions from Ext 4.2

fix asset path generation in scss
remove ExtJS 4.1 assets
Use ExtJS 4.2 assets for classic theme
include Sencha-provided Ext.ux files
Update README for ExtJS 4.2 and cleanup whitespace
Version bump for latest ExtJS
Merge branch 'extjs-4.2'