Import erlang-15.1.3.1 as wip/erlang15.

Riak won't support R16 until next release, this package holds
lang/erlang from just before it was updated to R16.
Add an explicit conflict on another erlang
Update riak to 1.4.0.

Riak won't support R16 until 1.5.x, so depending on a temporary
wip/erlang15 package. Riak actually embeds Erlang, so doesn't need
a runtime dependency.

Notable changes since 1.3.1:

* Improved Binary Format
* Changed behavior of riak attach
* riak-admin transfers Improvements
* Lager Upgrade 1.2.2 to 2.0.0
* Data Types: PN-Counters
* Improved Riak Control
* New command: riak-debug
* Packaging / Runtime changes (semi-relevant for pkgsrc)

See full release notes:

https://github.com/basho/riak/blob/master/RELEASE-NOTES.md