Move back warnings to Core

Related to https://github.com/CocoaPods/CocoaPods/issues/2028

Gemfile.lock

@@ -7,7 +7,7 @@ GIT
 
 GIT
   remote: https://github.com/CocoaPods/Core.git
-  revision: 134c8d4b2c8bcfba32d1e28766a2c8633b222d15
+  revision: a6d129d9a72bd7324414001daacbe8bfd96b2311
   branch: master
   specs:
     cocoapods-core (0.31.1)

lib/cocoapods.rb

@@ -72,24 +72,3 @@ if ENV['COCOA_PODS_ENV'] == 'development'
   # require 'awesome_print'
   # require 'pry'
 end
-
-# TODO remove for CocoaPods 0.31
-#
-module Pod
-  class Specification
-    def pre_install(&block)
-      UI.warn "[#{self}] The pre install hook of the specification " \
-        "DSL has been deprecated, use the `resource_bundles` or the " \
-        "`prepare_command` attributes."
-      UI.puts "[#{self}] The pre_install hook will be removed in the next release".red
-      @pre_install_callback = block
-    end
-    def post_install(&block)
-      UI.warn "[#{self}] The post install hook of the specification " \
-        "DSL has been deprecated, use the `resource_bundles` or the " \
-        "`prepare_command` attributes."
-      UI.puts "[#{self}] The post_install hook will be removed in the next release".red
-      @post_install_callback = block
-    end
-  end
-end