Simplify example

I saw that `uglify` was used twice in the readme example, once as a var and again as a function. So I updated to:

```js
var uglify = require('uglify-js')
var map = require('vinyl-map')
var gulp = require('gulp')

gulp.task('minify', function() {
  return gulp.src('index.js')
    .pipe(minify)
    .pipe(gulp.dest('./dist'))
})

var minify = map(function(code, filename) {
  return uglify.minify(code, { fromString: true })
})
```

But thought it could even be simpler with the proposed changes. Cheers!