In 1.4.0 we used Elasticsearch 2.3.5 and Jackson 2.8. This
combination was incompatible in some circumstances due to a change
in Jackson (gh-6508). With Elasticsearch 2.4 yet to be released at the
time, the only way to restore compatibility was to downgrade Jackson.

With the release of Elasticsearch 2.4 we have another option: revert
the Jackson downgrade and upgrade Elasticsearch instead. While we
normally wouldn't consider upgrading to a new minor version of a
dependency in a maintenance release we have to do something to restore
compatibility. The alternative is to downgrade Jackson but that will
affect more people (Jackson is more widely used than Elasticsearch)
and will lose some functionality that was new in Jackson 2.8 that
people may already be relying upon.

This commit restores the use of Jackson 2.8 – including the
2.8-specific dependency management – and upgrades to Elasticsearch 2.4

Closes gh-6868