Use string in gemspec instead of using constant

For some reason changing the load path break tests with Rails 3.1 and
Rails 3.2.

I can't understand why, so it is better to remove.

Author: Rafael Mendonça França

sprockets-rails.gemspec

@@ -1,9 +1,6 @@
-$:.unshift File.expand_path("../lib", __FILE__)
-require "sprockets/rails/version"
-
 Gem::Specification.new do |s|
   s.name = "sprockets-rails"
-  s.version = Sprockets::Rails::VERSION
+  s.version = "2.1.2"
 
   s.homepage = "https://github.com/rails/sprockets-rails"
   s.summary  = "Sprockets Rails integration"