Add cop for Capybara expectations set on current_path

Setting expectations on `current_path` in our Capybara feature
specs leads to flaky tests.  Switching to using
[Capybara's `have_current_path` matcher](http://www.rubydoc.info/github/teamcapybara/capybara/master/Capybara/RSpecMatchers#have_current_path-instance_method)
helps since it uses Capybara's
[waiting functionality](https://github.com/teamcapybara/capybara/blob/master/README.md#asynchronous-javascript-ajax-and-friends)
that ensures that preceding actions (like `click_link`) have
completed.

This cop tells you not to use `expect(current_path).to
match(/lol/)`, encouraging you to use the
`have_current_path` matcher instead.

lib/rubocop-rspec.rb

@@ -29,6 +29,7 @@
 require 'rubocop/cop/rspec/around_block'
 require 'rubocop/cop/rspec/be_eql'
 require 'rubocop/cop/rspec/before_after_all'
+require 'rubocop/cop/rspec/capybara/current_path_expectation'
 require 'rubocop/cop/rspec/capybara/feature_methods'
 require 'rubocop/cop/rspec/describe_class'
 require 'rubocop/cop/rspec/describe_method'

lib/rubocop/cop/rspec/capybara/current_path_expectation.rb

@@ -0,0 +1,30 @@
+module RuboCop
+  module Cop
+    module RSpec
+      module Capybara
+        # Checks for expectations set on `current_path`, where instead the
+        # `have_current_path` matcher (http://www.rubydoc.info/github/
+        # teamcapybara/capybara/master/Capybara/RSpecMatchers#have_current_path-
+        # instance_method) should be used on `page`, since it uses Capybara's
+        # waiting functionality (https://github.com/teamcapybara/capybara/blob/
+        # master/README.md#asynchronous-javascript-ajax-and-friends) which
+        # ensures that preceding actions (like `click_link`) have completed. 
+        class CurrentPathExpectation < Cop
+          MSG = 'Do not set an RSpec expectation on `current_path` in ' \
+                'Capybara feature specs - instead, use the ' \
+                '`have_current_path` matcher on `page`'
+
+          def_node_matcher :expectation_set_on_current_path, <<-PATTERN
+            (send nil :expect (send nil :current_path))
+          PATTERN
+
+          def on_send(node)
+            expectation_set_on_current_path(node) do |match|
+              add_offense(node, :selector)
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/rspec/capybara/current_path_expectation_spec.rb

@@ -0,0 +1,16 @@
+RSpec.describe RuboCop::Cop::RSpec::Capybara::CurrentPathExpectation do
+  subject(:cop) { described_class.new }
+
+  it 'flags violations for `expect(current_path)`' do
+    expect_offense(<<-RUBY)
+      expect(current_path).to eq("/callback")
+      ^^^^^^ Do not set an RSpec expectation on `current_path` in Capybara feature specs - instead, use the `have_current_path` matcher on `page`
+    RUBY
+  end
+
+  it 'doesn\'t flag a violation for other expectations' do
+    expect_no_offenses(<<-RUBY)
+      expect(current_user).to eq(user)
+    RUBY
+  end
+end