Fix failing npm install

Hey folks! Awesome work on this project, and thank you for open-sourcing it.

Currently, npm installs are failing due to the `postinstall` script:

```sh
npm http GET https://registry.npmjs.org/skatejs
npm http 304 https://registry.npmjs.org/skatejs

> skatejs@0.9.2 postinstall /Users/stevenschobert/Desktop/node_modules/skatejs
> ./build/bin/npm-postinstall.sh

sh: ./build/bin/npm-postinstall.sh: Permission denied
npm ERR! skatejs@0.9.2 postinstall: `./build/bin/npm-postinstall.sh`
npm ERR! Exit status 126
npm ERR!
npm ERR! Failed at the skatejs@0.9.2 postinstall script.
npm ERR! This is most likely a problem with the skatejs package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     ./build/bin/npm-postinstall.sh
npm ERR! You can get their info via:
npm ERR!     npm owner ls skatejs
npm ERR! There is likely additional logging output above.

npm ERR! System Darwin 13.3.0
npm ERR! command "/Users/stevenschobert/.nvm/v0.10.26/bin/node" "/Users/stevenschobert/.nvm/v0.10.26/bin/npm" "install" "skatejs"
npm ERR! cwd /Users/stevenschobert/Desktop
npm ERR! node -v v0.10.26
npm ERR! npm -v 1.4.3
npm ERR! code ELIFECYCLE
npm ERR!
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/stevenschobert/Desktop/npm-debug.log
npm ERR! not ok code 0
```

I updated the `npm-postinstall.sh` script to be non-executable, and then updated the `package.json` to execute the script using `sh`, rather than executing just the file.

This should fix the npm install error next time the package is published _(I hope)_.

Keep up the great work! :+1: