Research possible approaches to speeding up dependencies portion of CI builds

# Problem

Our `circle.yml` does the following at time of writing:

```
dependencies:
pre:
- |
if [[ ! -e ~/.yarn/bin/yarn || $(yarn --version) != "${YARN_VERSION}" ]]; then
curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version $YARN_VERSION
fi
cache_directories:
- ~/.yarn
- ~/.cache/yarn
- /home/ubuntu/nvm/versions/node/v6.9.4/bin
- /home/ubuntu/nvm/versions/node/v6.9.4/lib/node_modules
override:
- yarn global add bower
- yarn global add phantomjs-prebuilt
- yarn add node-sass
- yarn install
- bower install
```

The slowest steps of the `override` are as follows:
- `yarn global add bower` ~10s
- `yarn global add phantomjs-prebuilt` ~10s
- `yarn add node-sass` ~25s
- `yarn install` ~25s

Someone should research if there is any way to speed any of these steps up.

從 circle.yml 和其中列出的相依性覆寫命令開始。測量目前 yarn global add、node-sass、yarn install 和 bower install 的耗時，然後研究這些步驟的更快方法。完成的標準是記錄可行的選項，以及它們對 CI 建置時間的預期或實測影響。

由索引模型根據 Issue 內容生成。