fixes a few issues I was having, namely 'npm' not being in the path.

also insures that grunt + grunt-cli as well as bower are _actually_ installed
before calling them to do post-deploy builds.

It may seem dirty add the '--allow-root' and '--force' flags; here's why I did:
- sometimes (especially when debugging on DO droplets) users will be on the
  root account (I'm not saying they should be!); without this flag, bower will
  refuse to run. I don't see any harm in allowing it for 'git-push' style
  builds anyways...
- because I can't find any not-ridiculous way to re-push with "force enabled"
  short of issuing sketchy direct-commands to the container.