Update uniqueness.rb

I got an the following issue:

NoMethodError: undefined method `index_type' for nil:NilClass
	from /home/dre/.rvm/gems/jruby-1.7.4/bundler/gems/neo4j-8c5960b3e5e2/lib/neo4j/rails/validations/uniqueness.rb:15:in `setup'
	from org/jruby/RubyArray.java:1617:in `each'

when I used this kind of validation in the following conditions:

    rails (4.1.0.beta)
    neo4j (2.3.0-java)

And this updatefixes an issue.
Merge pull request #278 from borodiychuk/patch-1

Update uniqueness.rb