Fix #95: Update to rebar 2.5.1 and fix riak_pb_msgcodegen rebar plugin

* Update to rebar 2.5.1

* Fix riak_pb_msgcodegen rebar plugin to use the newer rebar plugin API.

Rebar now has separate hook names for post_* and pre_* hooks. This commit uses the pre_compileand pre_clean hooks.

This fixes #95