os/mac/xcode: fix CVS/GCC check if no Xcode

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.

Library/Homebrew/os/mac/xcode.rb

@@ -130,11 +130,11 @@ module OS
       end
 
       def provides_gcc?
-        version < "4.3"
+        installed? && version < "4.3"
       end
 
       def provides_cvs?
-        version < "5.0"
+        installed? && version < "5.0"
       end
 
       def default_prefix?