Remove crypto module deprecations

When compiling with R16B02 the following deprecation warnings were
encountered:

```
src/prf_crypto.erl:36: Warning: crypto:des_cbc_encrypt/3 is deprecated and will be removed in in a future release; use crypto:block_encrypt/4
src/prf_crypto.erl:43: Warning: crypto:des_cbc_decrypt/3 is deprecated and will be removed in in a future release; use crypto:block_decrypt/4
src/prf_crypto.erl:46: Warning: crypto:md5/1 is deprecated and will be removed in in a future release; use crypto:hash/2
```

Since I use `warnings_as_errors` erlc option I needed to work around these
deprecation warnings for newer versions of Erlang (R16B+). I did this using
a `rebar.config.script` and the Erlang preprocessor's `-ifdef` construct.
Therefore, it should work for previous versions as well as newer versions of
Erlang.

This does make one assumption, an assumption I believe to be "safe", but
worth noting anyway. It assumes that if `crypto:block_encrypt/4` is exported
from the `crypto` module that `crypto:block_decrypt/4` will also be exported.

Happy to amend that shortcut if necessary.