Manual merge of backwards-compatibility logic for "crypt:md5_xxx" functions in latest
develop branch (1.9) of git@github.com:basho/webmachine.git. This adds a file,
rebar.config.script, which runs a bit of erlang to determine whether the installed OTP
release on the host is  =< "R15B01" - if true this sets an ePP flag (old_hash) which
is used in src/webmachine_decision_core.erl to compile with the deprecated crypto MD5 function set;
otherwise (with more recent OTP releases) the new more generalized "hash_xxx" versions are used.
This builds as expected on R15B01 (old crypto functions) and R16B03 (new crypto functions).