Transform list of unicode chars in templater

Since OTP 17 strings read from a file are by default parsed as a list of
unicode characters.
If such a list contains a non-ascii character (unicode > 255) then a
call to erlang:list_to_binary/1 fails.
This commit converts unicode list of chars to binary via unicode module
if ran against OTP >= 17.
As far as I can see it's used in a similar fashion only in the ``rebar_otp_release_tests`` test, the returned value is verified further.
How about extracting this R/post-R check into a local function so it can be easily extracted from the module if necessary?