Fix app.css compilation by improving bower function in ds script.

#### What's this PR do?

- Fixes `app.css` compilation issue:  
  
  ```
  Running "sass:prod" (sass) task
  Warning: /var/www/vagrant/lib/themes/dosomething/paraneue_dosomething/scss/app.scss:17: error: file to import not found or unreadable: "../bower_components/bourbon/dist/bourbon"
  Use --force to continue.
  Aborted due to warnings.
  ```
- Enhances `bower()` function in `ds` script by making it decide automatically whether `bower` packages should be installed or updated

#### How should this be manually tested?
Rebuild your environment as usual:
- `vagrant destroy -f`
- `vagrant up`
- `vagrant ssh`
- `setup-www`
- `cd /var/www/vagrant`
- `bin/ds build`
- Wait for `Installing NPM dependencies...` task and see if it fails

#### Any background context you want to provide?
Suddenly bower stopped download modules to `bower_components` during `update` task, now it does it only on `install`.

I can't figure a reason why and where it started. Here's the list with my suspects:
 - last changes to dosomething vagrant box be me
 - last release of the neue by @DFurnes 
 - changes in the `bower` itself