Remove the have extension

This functionality was extracted into
rspec/rspec-collection_matchers#5

Fixes #808

Author: soulcutter

lib/rspec/rails/matchers.rb

@@ -22,6 +22,5 @@ class AssertionFailedError < StandardError
 require 'rspec/rails/matchers/routing_matchers'
 require 'rspec/rails/matchers/be_new_record'
 require 'rspec/rails/matchers/be_a_new'
-require 'rspec/rails/matchers/have_extension'
 require 'rspec/rails/matchers/relation_match_array'
 require 'rspec/rails/matchers/be_valid'