add "since" doc metadata to System.otp_release/0

[adrianomitre]

$ docker run -it elixir:1.3.0 bash
root@983045508faf:/# elixir -e "IO.puts System.otp_release"
19

$ docker run -it elixir:1.2.6 bash
root@af8db535bb1e:/# elixir -e "IO.puts System.otp_release"
** (UndefinedFunctionError) undefined function System.otp_release/0
    (elixir) System.otp_release()
    (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
    (stdlib) erl_eval.erl:878: :erl_eval.expr_list/6
    (stdlib) erl_eval.erl:404: :erl_eval.expr/5
    (elixir) lib/code.ex:168: Code.eval_string/3

References:

• 5533cc4 - v1.3.0
• ea119f4 - v1.4.0 onwards