Don't require npm install node-slate if dep

[OmgImAlexis]

I've already got node-slate added as a dep in my project but I this module keeps installing node-slate anyway.

➜  template git:(master) ✗ yarn add -D sdelements/node-slate
yarn add v1.3.2
[1/5] 🔍  Validating package.json...
[2/5] 🔍  Resolving packages...
[3/5] 🚚  Fetching packages...
[4/5] 🔗  Linking dependencies...
[5/5] 📃  Building fresh packages...
success Saved lockfile.
success Saved 1 new dependency.
└─ node-slate@1.0.0
✨  Done in 37.36s.
➜  template git:(master) ✗ yarn gulp slate                  
yarn run v1.3.2
$ /Users/xo/code/lad/template/node_modules/.bin/gulp slate
[13:54:12] Using gulpfile ~/code/lad/template/gulpfile.js
[13:54:12] Starting 'slate'...
settings: { source: 'source', build: 'build' }
downloading...
npm install ./node_modules/node-slate

[pilafmon]

I believe the problem is due to node-slate not being published on npm.

See:
sdelements/node-slate/issues/4

This project defines the dependency to node-slate using a specific GitHub commit rather than an npm version number. Do you know if there's a better way to define the dependency to workaround the problem?