Set max json request body size with EXPRESS_LIMIT

[pearswj]

Extends #1106 to include JSON requests.

Background: I pipe GitHub "push" events to hubot (using content-type: application/json) and every now and again express returns 413 Payload Too Large if the push contains a lot of commits. I found the new EXPRESS_LIMIT env var, but it only applies to urlencoded requests. The name of this env var suggests that it's not specific to any one content type in particular, so I'm hoping it's OK to apply this limit to JSON requests too! No new documentation required, as the current wording doesn't specifically mention urlencoded requests.

[ilyoff]

Any progress here? I just need this changes. Or can I make same PR by myself?