[erlang] Lower the required NIF version and simplify

[cocoa-xu]

Erlang NIF should be backwards compatible so we can try to lower the required NIF version, say NIF 2.15, and we can simply do a NIF 2.15 release for all variants instead of releasing one for each NIF version. In this way, this library will be available to more users who're using old major versions of Erlang/OTP.

[cocoa-xu]

We can do this after addressing #210 which could bring less burden.