do not use `_()` in rt-class helper

[nippur72]

rt-class now results in a call to _() (lodash function) to be generated.

This cause two possible issues:

• templates not working due to incorrect lodash import with babel (as discussed in fixed rt-class to also work with lodash 4.5.0 #110)
• it's no longer possible to isolate the only lodash methods used by react-templates (e.g. by making a custom build of lodash).

This can be fixed easily by changing _(...).transform( into _.transform(...,.