Fix build error on Rubinius by requiring matcher

There was some problem with class autoloading in Rubinius, which can be
workaround by just requiring that missing file.

See rubinius/rubinius#2934

spec/appraisal/file_spec.rb

@@ -1,6 +1,10 @@
 require 'spec_helper'
 require 'appraisal/file'
 
+# Requiring this to make the test pass on Rubinius 2.2.5
+# https://github.com/rubinius/rubinius/issues/2934
+require 'rspec/matchers/built_in/raise_error'
+
 describe Appraisal::File do
   it "should complain when no Appraisals file is found" do
     ::File.stub(:exist?).with("Appraisals").and_return(false)