Fix crash due to spot instance race condition

Rubber crashed on me creating spot instances. It seems to be a race condition in lib/rubber/recipes/rubber/instances.rb 
Here is the error:
/Users/jimsalem/.rvm/gems/ruby-2.1.2@master/bundler/gems/rubber-6b301f4d3fda/lib/rubber/recipes/rubber/instances.rb:306:in `create_instance': undefined method `[]' for nil:NilClass (NoMethodError)
Note that line 306 is line 295 in the current master.

This one line change fixes the error.