[ci skip] Add `Thread.current` to match internals

We call the thread variable accessors on `Thread.current`, which matches Ruby's 
documentation:

http://ruby-doc.org/core-2.2.0/Thread.html#method-i-thread_variable_get

Fix these to stay `current` ( ͡° ͜ʖ ͡°)

activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb

@@ -15,7 +15,7 @@ class Module
   #   end
   #
   #   Current.user # => nil
-  #   Thread.thread_variable_set("attr_Current_user", "DHH")
+  #   Thread.current.thread_variable_set("attr_Current_user", "DHH")
   #   Current.user # => "DHH"
   #
   # The attribute name must be a valid method name in Ruby.
@@ -63,7 +63,7 @@ class Module
   #   end
   #
   #   Current.user = "DHH"
-  #   Thread.thread_variable_get("attr_Current_user") # => "DHH"
+  #   Thread.current.thread_variable_get("attr_Current_user") # => "DHH"
   #
   # If you want to opt out the instance writer method, pass
   # <tt>instance_writer: false</tt> or <tt>instance_accessor: false</tt>.