Added jRuby support

To be used this gem in jRuby we will be using
activerecord-jdbcpostgresql-adapter or maybe another adapter like
activerecord-postgis-adapter. They all both support hstore because they
run over postgres anyway.

.travis.yml

@@ -2,3 +2,5 @@ language: ruby
 rvm:
   - 1.9.3
   - ruby-head
+  - jruby-1.7
+  - jruby-9.0.4.0

multilang-hstore.gemspec

@@ -19,7 +19,9 @@ Gem::Specification.new do |s|
     "spec/schema.rb",
     "spec/spec_helper.rb"
   ]
-  s.add_dependency 'pg', '~> 0.0'
   s.add_dependency 'activerecord', '~> 4.0'
+  s.add_dependency 'activerecord-jdbcpostgresql-adapter', :platform => :jruby
+  s.add_dependency 'pg', '~> 0.0', :platform => :ruby
+  end
 end