ccb11935f6
On systems prior to 10.9, formulae that use CVS as a download source check whether the installed Xcode already provides CVS to avoid adding a dependency on the `cvs` formula. Unfortunately, if no Xcode is installed the check fails with undefined method `<' for nil:NilClass causing the formula to become unloadable. This in turn causes some taps to be untappable since #396 added the `readall` check on `tap`. Closes #508.