Fixes rbenv on OpenBSD and any other systems that don't support head -c

OpenBSD doesn't support 'head -c N'. See http://www.openbsd.org/cgi-bin/man.cgi?query=head&apropos=0&sektion=0&manpath=OpenBSD+Current&arch=i386&format=html

Using 'cut' with a byte range accomplishes the same thing.