Don't include the JAR file in the non-Java gem

Closes #31.

Author: sferik

thread_safe.gemspec

@@ -8,7 +8,8 @@ Gem::Specification.new do |gem|
   gem.summary       = %q{A collection of data structures and utilities to make thread-safe programming in Ruby easier}
   gem.homepage      = "https://github.com/headius/thread_safe"
 
-  gem.files         = `git ls-files`.split($\) + ['lib/thread_safe/jruby_cache_backend.jar']
+  gem.files         = `git ls-files`.split($\)
+  gem.files        += ['lib/thread_safe/jruby_cache_backend.jar'] if defined?(JRUBY_VERSION)
   gem.platform      = 'java' if defined?(JRUBY_VERSION)
   gem.executables   = gem.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})