bower update unconditionally reinstalls all packages

[nschloe]

When trying to get caching on travis to work for my front-end packages, I found that bower update doesn't do what I expected it to do, namely to

1. Check which packages and versions are installed under bower_components and
2. Update the ones that don't match the specs in bower.json.

Instead, what happens is that all packages are overridden by whatever is specified in bower.json, no matter if they are up-to-date or not. This results in our travis tests taking 30% longer than necessary.

Any comments, opinions, hints here?

[sheerun]

That's one of the oldest open issues bower has: #256

This behavior needs to be tested and fixes, but we don't have resources to do so.

Feel free to help and send PR with fix :)

A failing test for this behavior can be added to:
https://github.com/bower/bower/blob/master/test/commands/update.js