Installation script/docs out of date

The links to CCP4 tarballs used in `test_build.sh` ([linux example](http://devtools.fg.oisin.rc-harwell.ac.uk/nightly/ccp4-linux64-latest.tar.bz2)) do not work (404 Not Found). Downloading the appropriate tarball from [https://www.ccp4.ac.uk/download](https://www.ccp4.ac.uk/download/#os=mac) and manually extracting before proceeding with the build script worked for me.