Replace postinstall script with Heroku's postbuild script

[gemmaleigh]

What problem does the pull request solve?

The postinstall script is causing the npm release of govuk-elements-sass to fail.

Since this script was added to build assets for Heroku, instead use
a Heroku-specific build step.

Replace postinstall with heroku-postbuild, which runs after Heroku installs dependencies.

What type of change is it?

• Bug fix (non-breaking change which fixes an issue)

Has the documentation been updated?

• I have read the CONTRIBUTING document.

[gemmaleigh]

-----> Deleting 1 files matching .slugignore patterns.
-----> Node.js app detected
-----> Creating runtime environment
       
       NPM_CONFIG_LOGLEVEL=error
       NPM_CONFIG_PRODUCTION=false
       NODE_VERBOSE=false
       NODE_ENV=development
       NODE_MODULES_CACHE=true
-----> Installing binaries
       engines.node (package.json):  >=8.1.4
       engines.npm (package.json):   unspecified (use default)
       
       Resolving node version >=8.1.4 via semver.io...
       Downloading and installing node 8.1.4...
       Using default npm version: 5.0.3
-----> Restoring cache
       Skipping cache restore (new-signature)
-----> Building dependencies
       Installing node modules (package.json)
       
       > [email protected] install /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d/node_modules/node-sass
       > node scripts/install.js
       
       Downloading binary from https://github.com/sass/node-sass/releases/download/v4.5.3/linux-x64-57_binding.node
       Download complete
       Binary saved to /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d/node_modules/node-sass/vendor/linux-x64-57/binding.node
       Caching binary to /app/.npm/node-sass/4.5.3/linux-x64-57_binding.node
       
       > [email protected] install /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d/node_modules/fsevents
       > node install
       
       
       > [email protected] postinstall /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d/node_modules/node-sass
       > node scripts/build.js
       
       Binary found at /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d/node_modules/node-sass/vendor/linux-x64-57/binding.node
       Testing binary
       Binary is fine
       added 849 packages in 71.029s
       Running heroku-postbuild
       
       > govuk-elements@ heroku-postbuild /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d
       > gulp build
       
       [14:47:17] Using gulpfile /tmp/build_8bab95e8869563c6dfe9170452a7733b/alphagov-govuk_elements-c2e988d/gulpfile.js
       [14:47:17] Starting 'build'...
       [14:47:17] Starting 'clean'...
       [14:47:17] Finished 'clean' after 8.11 ms
       [14:47:17] Starting 'styles'...
       [14:47:17] Starting 'images'...
       [14:47:17] Starting 'scripts'...
       [14:47:17] Finished 'scripts' after 105 ms
       [14:47:17] Finished 'images' after 508 ms
       [14:47:19] Finished 'styles' after 1.69 s
       [14:47:19] Finished 'build' after 1.7 s
-----> Caching build
       Clearing previous node cache
       Saving 2 cacheDirectories (default):
       - node_modules
       - bower_components (nothing to cache)
-----> Build succeeded!
-----> Discovering process types
       Procfile declares types -> web
-----> Compressing...
       Done: 30.7M
-----> Launching...
       Released v4
       https://govuk-elements-review-pr-530.herokuapp.com/ deployed to Heroku