Also, make dev/update_packages.dart skip directories that have no
pubspec.yaml, since firebase/ doesn't have one.

Makes it more obvious where we are spending our time on Travis.

Also added travis caching for the global pub cache to
speed this up!  Modeled what dartdoc does:
https://github.com/dart-lang/dartdoc/blob/master/.travis.yml#L13

@Hixie

Add an --upgrade flag to the dev/update_packages.dart flag which runs
'pub upgrade' instead of 'pub get'. Tell people to use this when using
'flutter analyze' since 'pub get' doesn't have the same guarantee of
getting everything in sync.

Should make Travis green again. Also, add better logging to
dev/update_packages.dart.

Make it possible to run dev/update_packages.dart directly from the shell.

@abarth