use full path in integration tests

looks like this is required after updating to actionpack 4.2.0

See change made in this commit:
rails/rails@4fd144d?diff=unified#diff-82ecef623242ece0e7d717577485d988L303

Author: Jordan Stephens

test/jsxtransform_test.rb

@@ -21,13 +21,13 @@
 class JSXTransformTest < ActionDispatch::IntegrationTest
 
   test 'asset pipeline should transform JSX' do
-    get 'assets/example.js'
+    get '/assets/example.js'
     assert_response :success
     assert_equal EXPECTED_JS, @response.body
   end
 
   test 'asset pipeline should transform JSX + Coffeescript' do
-    get 'assets/example2.js'
+    get '/assets/example2.js'
     assert_response :success
     # Different coffee-script may generate slightly different outputs,
     # as some version inserts an extra "\n" at the beginning.