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"*. These consistently failed
since the ring resizing change was merged.

Since `.travis.yml` was updated last (a year ago) R15B0{2-3} and R16B
were released. R16B01 and R16B02 to not appear to be offered as choices
in Travis CI yet ([according to Travis CI's documentation]
(http://about.travis-ci.org/docs/user/languages/erlang/)).