Fix rebar plugin and update to rebar 2.5.1

* 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_compile`and `pre_clean` hooks.

This fixes https://github.com/basho/riak_pb/issues/95