Update otp_release list in .travis.yml

Changes:
* Removed R14* Erlang version targets from .travis.yml
* Added R15B* and R16B Erlang version targets to .travis.yml

Background:
Until this change Travis CI was building against Erlang R14 which is not
supported for Riak (KV) according to [Basho documentation]
(http://docs.basho.com/riak/latest/ops/building/installing/erlang/),
which states, *"Riak requires Erlang R15B01"*.