update npm modules

- browserify v6 (could not upgrade past v7 due to an issue with
remapify)
- watchify 2.1.1
- gulp-autoprefixer 2.0.0
- gulp-filter 2.0.0
- gulp-flatten 0.0.4
- gulp-load-plugins 0.8.0
- gulp-sass 1.2.4
- jshint-stylish 1.0.0
- remapify 1.3.0