Add MacRuby note to gemspec summary

cocoapods.gemspec

@@ -6,7 +6,7 @@ Gem::Specification.new do |s|
   s.version  = Pod::VERSION
   s.date     = "2011-09-17"
   s.license  = "MIT"
-  s.summary  = "A simple Objective-C library package manager."
+  s.summary  = "A simple Objective-C library package manager. (Requires MacRuby.)"
   s.email    = "eloy.de.enige@gmail.com"
   s.homepage = "https://github.com/alloy/cocoapods"
   s.authors  = ["Eloy Duran"]