Updating Jackson to 2.4.0

Minor release with new functionality but backwards compatible.

- Official support for Guava HostAndPort (removed Dropwizards deserializer)
- Jdk7Module with support for Path
- Bunch of misc improvements

https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.4

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It looks like Jackson 2.4.0 actually has a [couple regressions](https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.4.1) from 2.3.x, so it may be worth holding off merging this until 2.4.1 is out and going straight to that. Thoughts?
It's related - ObjectNode.put() was [deprecated in 2.4](https://github.com/FasterXML/jackson-databind/blob/jackson-databind-2.4.1/src/main/java/com/fasterxml/jackson/databind/node/ObjectNode.java#L417). ObjectNode.set() has the same behaviour, but returns a reference to the ObjectNode rather than the replaced value.