Use bower update to avoid cleaning caches

When a bower dependency is aligned on a branch (ex: nuxeo/nuxeo-elements#master) bower install is using the version from the local cache without updating to the latest commit.
According to discussions on several bower issues ( #1242, #994, ...) using "bower update" should force an update check on the repository.
We should also force resolution to latest version with "-F" to avoid manual resolutions when local versions are out of date.