Allow rails 4 beta testing

EDIT: Will need to be pulled into a rails4 branch to fix other issues like this:

````
uninitialized constant ActiveModel::Observer
org/jruby/RubyModule.java:2677:in `const_missing'
/Users/pboling/.rvm/gems/jruby-1.7.2@rails4/bundler/gems/neo4j-edce5343e8cd/lib/neo4j/rails/observer.rb:121:in `Rails'
/Users/pboling/.rvm/gems/jruby-1.7.2@rails4/bundler/gems/neo4j-edce5343e8cd/lib/neo4j/rails/observer.rb:2:in `Neo4j'
````