Add a dependency update command

[treeder]

eg: fn dep to run:

• dep ensure
• bundle update
• etc.

[rdallman]

this creates kind of a slippery slope to do mvn install, bundle install, npm install, etc... and we become the owners of dependency management across every language.

if the image itself e.g. for go has a dep ensure command for go (that skips cache) that runs when the Gopkg.toml file changes, we should be covered with fn build / fn deploy right? we just have to make sure the file adding happens before dep ensure so that the docker cache gets skipped, I presume the tradeoff is here alone.