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.