Change plugin name and depend on 2.4

As we get closer to releasing this plugin to work with 2.4, we should
update the name -- going with search-processor -- to better reflect the
idea that it will be able to transform search requests and responses.
Also, we need to update the build to point to 2.4 snapshots, instead of
2.1.

Signed-off-by: Michael Froh <froh@amazon.com>