subversion: `install --ruby` fails if brewed ruby is present

Running `brew install subversion --ruby` will fail if there is a brew-installed ruby 1.9, because subversion requires 1.8 and it's finding the 1.9.

The attached fix sets a few paths explicitly to the system ruby location, and sets a more specific ruby library version so that 1.8 will get used. This allows the install to succeed.