Fixes*  "npm not found" and random plugin-not-running issues

fixes* a few issues I was having, namely due to `npm` not being in `$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.

*fixes == command-line hacks from a guy that started using docker and dokku _today_. There are probably better ways to do this. I just need bower and grunt to build when I push and this accomplishes that (for me). 

__If you are still having one of these issues, comment here or message me with more details__ (https://github.com/thrashr888/dokku-bower-grunt-build-plugin/issues/3 and https://github.com/thrashr888/dokku-bower-grunt-build-plugin/issues/4).
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Yeah... this is a little sketch. It's a mashup of [this one-liner](https://github.com/progrium/dokku/issues/380#issuecomment-30617145) by @alexanderbeletsky and [this local node_modules tip](http://stackoverflow.com/a/15157360) on SO.