Order JSON nodes more reliably

[aripollak]

This was originally opened as #171 but was closed based on the assumption that hashes are inherently unordered. However, the Hash class in recent versions of Ruby automatically keeps track of key insertion order by default, so in theory this should be easier to implement now.
Rails' to_json orders things consistently, so it would be nice if rabl did too. It's easier to work with JSON output that's deterministically ordered for testing purposes, so that e.g. fixture files don't show differences with every new run.

[tibbon]

+1

[pibo]

+1

[marine44]

+1

[sskylar]

+1