Make use of HUBOT_PATH env variable when loading scripts

Currently, you have to be in a proper working directory to launch Hubot. At the same time, we have defined HUBOT_PATH we are almost not using at all.

After that patch, you would be able to execute Hubot from any directory by providing proper HUBOT_PATH:

```sh
BASEDIR=/absolute/path/to/bot

npm install --prefix $BASEDIR

env HUBOT_PATH="$BASEDIR" $BASEDIR/node_modules/.bin/hubot
```