…r to use text type with ES 5+

The ES connector now queries for the version.  The version is stored in
an instance of ElasticsearchClient, which is passed around so that
it can be queried at the appropriate times (such as when inferring
schema mappings).

As part of this change, the old Jest client is now wrapped by a
higher-level client object.  Also, all the Jest dependencies have been
isolated to the io.confluent.connect.elasticsearch.jest package.  This
makes the third-party dependencies more clear and will facilitate moving
to a different ES client library in the future should we choose to do
so.

These changes have been tested with ES 2.x, 5.x, and 6.x.