Replace babel with bublé

Pros of bubel over babel:

- No configuration (a la standard)
- Runs twice as fast, for quicker development
- Converts everything to ES5 (which is likely to be faster than ES6,
untested)
- Easy to swap Babel back in -- low commitment

Cons:
- Less battle-tested than Babel, but recommended by React core
developer so probably not too bad
- No babel plugin support, but we're not using that right now anyway.
Can switch back to babel if we need that later

BEFORE:

$ time npm run build

> webtorrent-desktop@0.16.0 build /Users/feross/code/webtorrent-desktop
> babel --quiet src --out-dir build

npm run build  3.07s user 0.27s system 115% cpu 2.902 total

AFTER:

$ time npm run build

> webtorrent-desktop@0.16.0 build /Users/feross/code/webtorrent-desktop
> buble src --output build

npm run build  1.38s user 0.16s system 114% cpu 1.343 total

bin/package.js

@@ -39,9 +39,9 @@ function build () {
   rimraf.sync(DIST_PATH)
   rimraf.sync(BUILD_PATH)
 
-  console.log('Babel: Building JSX...')
+  console.log('Build: Transpiling to ES5...')
   cp.execSync('npm run build', { NODE_ENV: 'production', stdio: 'inherit' })
-  console.log('Babel: Built JSX.')
+  console.log('Build: Transpiled to ES5.')
 
   var platform = argv._[0]
   if (platform === 'darwin') {