Update README for new tap location

Github repository ownership has changed from edc to rdkit.
Remove version that is redundant with version scanned from URL

Based on warning from `brew audit`: Stable: version 2013.09.2 is redundant with version scanned from URL
Switch to tar.gz download instead of zip

Based on warning from `brew audit`: Use GitHub tarballs rather than zipballs
Clarify swig dependency as build only
Use preferred head url definition style
Use build instead of ARGV to check options