Pretty json

@mattgillooly 

create the definitions dir if it doesnt exist, add option to pretty print the defintion.json via the PRETTY env:

    rake defintion PRETTY=true
Rake has a `directory` task that accomplishes this in one line.  http://docs.seattlerb.org/rake/Rake/DSL.html#method-i-directory

This is even defined via `directory path` on line 37.  https://github.com/swipely/pipely/blob/pretty_json/lib/pipely/tasks/definition.rb#L37

What's missing is the linkage between `task name` and `directory path` as its prerequisite on line 40.