Dependency: use instanceof? in eql?

This matches the eql? definition for requirements.

Library/Homebrew/dependency.rb

@@ -20,7 +20,7 @@ class Dependency
   end
 
   def eql?(other)
-    other.is_a?(self.class) && hash == other.hash
+    instance_of?(other.class) && hash == other.hash
   end
 
   def hash