Squashed 'vendor/rack-pjax/' changes from 092530e..ae6deee

ae6deee Do not overwrite X-PJAX-URL
c315fd0 Update install instructions
3af7b18 Point to non-legacy heroku app
070671b Do not overwrite X-PJAX-URL

git-subtree-dir: vendor/rack-pjax
git-subtree-split: ae6deee7dd58e4c8fc1fea8b590ec717e55265bd
Merge commit '44fcefc3fe77b4d044de6f71144d233a84277d01' into sample-app