ruby syntax error

fix the error when trying to run `rake formbuilder:install:migrations` from formbuilder-rb by my side:

```
/usr/local/var/rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/activesupport-4.1.6/lib/active_support/dependencies.rb:247:in `require':     /usr/local/var/rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/erector-rails4-0.2.0/lib/erector/caching.rb:13: syntax error, unexpected modifier_if
          dynamic_keys: if options[:needs_keys]
```