Fix issues building static assets

[davidfischer]

This makes a few changes to how our static assets are built that fixes some version incompatibility issues.

• Run gulp through an NPM script which ensures that the version from node_modules/.bin/ is used
• Upgrade vinyl and gulp-uglify

Testing

After pulling these changes, you'll need to run

$ npm install               # updates dependencies
$ npm run build             # outputs compiled files

After running this, git status should show the following as modified (at least):

modified:   readthedocs/builds/static/builds/js/detail.js
modified:   readthedocs/core/static/core/js/readthedocs-doc-embed.js
modified:   readthedocs/core/static/core/js/site.js
modified:   readthedocs/gold/static/gold/js/gold.js
modified:   readthedocs/projects/static/projects/js/import.js
modified:   readthedocs/projects/static/projects/js/tools.js

I tested this under a couple different Node/NPM version combinations:

• Node v10.5.0 with NPM 6.1.0 (latest released version)
• Node v8.11.3 with NPM 5.6.0 (latest LTS version)

Fixes #4280

[agjohnson]

Looks great! Thanks for digging into the error.

Super trivial, but for consistency, our tox js lint step could be updated to use the npm call as well.

[ericholscher]

Confirmed it's working here, too. 🎆

[davidfischer]

Super trivial, but for consistency, our tox js lint step could be updated to use the npm call as well.

I did this in #4312.