Revert "Merge pull request #40 from alphagov/add-placeholder-support"

This reverts commit c51b90dfb6e8bf45adbcea6d9e2957548c8233c7, reversing
changes made to d0a56cae590e3b12d90541c8c2b7c2d6d35f6925.

Deployment to Heroku is failing with this error:

npm ERR! node-sass@1.0.1 install: `node build.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the node-sass@1.0.1 install script.
npm ERR! This is most likely a problem with the node-sass package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node build.js
npm ERR! You can get their info via:
npm ERR!     npm owner ls node-sass
npm ERR! There is likely additional logging output above.
npm ERR! System Linux 3.8.11-ec2
npm ERR! command "/tmp/build_1acd273e-60b6-48c0-be8e-5bccbf9e4b97/vendor/node/bin/node" "/tmp/build_1acd273e-60b6-48c0-be8e-5bccbf9e4b97/vendor/node/bin/npm" "install" "--userconfig" "/tmp/build_1acd273e-60b6-48c0-be8e-5bccbf9e4b97/.npmrc" "--production"
npm ERR! cwd /tmp/build_1acd273e-60b6-48c0-be8e-5bccbf9e4b97
npm ERR! node -v v0.10.32
npm ERR! npm -v 1.4.28
npm ERR! code ELIFECYCLE
npm ERR! not ok code 0

Reverting update to node-sass on master until this is resolved.