Use correct SPDX ID for Apache license

See http://guides.rubygems.org/specification-reference/#license= and http://spdx.org/licenses/

Author: reefdog

closure-compiler.gemspec

@@ -2,7 +2,7 @@ Gem::Specification.new do |s|
   s.name      = 'closure-compiler'
   s.version   = '1.1.11'            # Keep version in sync with closure-compiler.rb
   s.date      = '2014-07-30'
-  s.license   = 'Apache 2.0'
+  s.license   = 'Apache-2.0'
 
   s.homepage    = "http://github.com/documentcloud/closure-compiler/"
   s.summary     = "Ruby Wrapper for the Google Closure Compiler"