Add node shebang to single JS builds

[eknkc]

Summary

Adds #!/usr/bin/env node at the top of single JS builds so they can be run directly if execute permissions are set. Fixes #888

Test plan

npm run test has no changes.

[bestander]

@eknkc, we don't have unit tests for this feature yet.
Could you use this Test Plan:

node ./scripts/build-webpack.js
./dist/yarn-v<version>.js -V

And confirm that it works

[eknkc]

@bestander

Naively testing the script works:

$ ./scripts/build-webpack.js
$ ./dist/yarn-0.15.1.js -V
0.15.1

Tried it with reinstalling own packages;

$ ./scripts/build-webpack.js
$ ./dist/yarn-0.15.1.js                                                                 
yarn install v0.15.1
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 📃  Building fresh packages...
success Saved lockfile.
✨  Done in 9.42s.

[bestander]

Great! thanks for the the PR, @eknkc