brew-doctor: use String.start_with?

Library/Homebrew/cmd/doctor.rb

@@ -17,7 +17,7 @@ class Volumes
   def which path
     @volumes.each_index do |i|
       vol = @volumes[i]
-      return i if is_prefix?(vol[1], path)
+      return i if vol[1].start_with? path.to_s
     end
 
     return -1
@@ -25,12 +25,6 @@ class Volumes
 end
 
 
-def is_prefix? prefix, longer_string
-  p = prefix.to_s
-  longer_string.to_s[0,p.length] == p
-end
-
-
 def remove_trailing_slash s
   (s[s.length-1] == '/') ? s[0,s.length-1] : s
 end