fix Capfile (remove capistrano/faster_assets)

[esetomo]

capistrano/faster_assets has been removed from Gemfile, but it remains in Capfile.

[esetomo]

after fix Capfile, another error on deploy:assets:precompile.

Webpacker is installed 🎉 🍰
Using /home/mastodon/app/releases/20170503081937/config/webpack/paths.yml file for setting up webpack paths
Compiling webpacker assets 🎉
/home/mastodon/app/shared/~/css-loader?{"minimize":true}!/home/mastodon/app/shared/~/postcss-loader!/home/mastodon/app/shared/~/sass-loader/lib/loader.js!/home/mastodon/app/shared/~/font-awesome/css/font-awesome.css
Module build failed: Error: No PostCSS Config found in: /home/mastodon/app/shared/node_modules/font-awesome/css
    at /home/mastodon/app/shared/node_modules/postcss-load-config/index.js:51:26
 @ /home/mastodon/app/shared/~/font-awesome/css/font-awesome.css 4:14-143
 @ ./app/javascript/packs/application.js
/home/mastodon/app/shared/~/font-awesome/css/font-awesome.css
Module build failed: ModuleBuildError: Module build failed: Error: No PostCSS Config found in: /home/mastodon/app/shared/node_modules/font-awesome/css
    at /home/mastodon/app/shared/node_modules/postcss-load-config/index.js:51:26
    at runLoaders (/home/mastodon/app/shared/node_modules/webpack/lib/NormalModule.js:192:19)
    at /home/mastodon/app/shared/node_modules/loader-runner/lib/LoaderRunner.js:364:11
    at /home/mastodon/app/shared/node_modules/loader-runner/lib/LoaderRunner.js:230:18
    at context.callback (/home/mastodon/app/shared/node_modules/loader-runner/lib/LoaderRunner.js:111:13)
    at /home/mastodon/app/shared/node_modules/postcss-loader/index.js:148:13

workaround:
$ echo 'module.exports = {};' > /:deploy_to/shared/postcss.config.js

[mjankowski]

Is there any sort of capistrano linter or check we could have had in place which would catch this on CI?