Display `if` block/lambda instead of `__` placeholder

[chaadow]

Using RubyVM ast new API, we can simple get a hold of the source of lambda/block.

This replaces the __ placeholder by the actual implementation of the @if lambda

[chaadow]

@ankane I guess this one should be closed as well :/

If so, it is unfortunate, because it really helps to know that some jobs are conditional ( with the if statement )

[chaadow]

Implemented this in https://github.com/chaadow/clockwork_web_plus