Support Erlang R15B

[ngocdaothanh]

Erlang R15B has been released.
Should this line in rebar.config:
{require_otp_vsn, "R14"}
be changed to:
{require_otp_vsn, "R15B"}?

[ronalfei]

I think you can try this: {require_otp_vsn, "R1[4|5]B"}