fix bin/rubocop to work when pwd != rails root

Change-Id: Ib8aabe6948800e0ab143b448e4814c916029668b
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/276296
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

bin/rubocop

@@ -10,13 +10,15 @@
 
 require "bundler/inline"
 
-# be tricksy and use a tiny subset of the gemfile so that
-# it will be very quick
-gemfile do
+Dir.chdir(File.expand_path("..", __dir__)) do
+  # be tricksy and use a tiny subset of the gemfile so that
+  # it will be very quick
+  gemfile do
     source 'https://rubygems.org/'
 
     gemfile = File.expand_path("../Gemfile.d/rubocop.rb", __dir__)
     eval(File.read(gemfile))
+  end
 end
 
 load Gem.bin_path("rubocop", "rubocop")