Kill nopt with fire

#### Overview

The main goal of this PR is to remove the GH tarball from the dependencies.  I chose to move from `nopt` to `commander.js` as suggest in https://github.com/Filirom1/findup/issues/2.  I noticed that the build was failing on Travis CI, so I implemented some changes to fix that (let me know if you prefer those in a separate PR).  Finally, I noticed that the build was failing on Travis because node 0.6 has `path.exists` rather than `fs.exists` so I implemented a check that uses the correct one to support node 0.6 (I am also happy to remove this at request).

I am happy to create a squashed commit in a separate PR at request.

#### Changes

* Replaced `nopt` fork with `commander.js`
* Updated Travis CI config to match package.json node engine spec
* Fallback to deprecated `path.exists` if needed (node 0.6 compatibility)
    * This can be removed if 0.6 compat is not desired

#### Suggested Manual Testing

Manually run through all the automated tests in `test/findup-tests.js` using the CLI.

Closes #2