Clean up npm scripts a bit more.

`docs-serve` was duplicated. Also, extend the existent `docs-serve` script for `docs-github-serve`.

package.json

@@ -59,10 +59,9 @@
     "coveralls": "shx cat js/coverage/lcov.info | coveralls",
     "docs": "npm-run-all --parallel css-docs js-docs --sequential docs-compile docs-lint",
     "docs-compile": "bundle exec jekyll build",
-    "docs-serve": "bundle exec jekyll serve",
     "postdocs-compile": "npm run docs-workbox-precache",
     "docs-github": "shx echo \"github: true\" > twbsconfig.yml && npm run docs-compile -- --config _config.yml,twbsconfig.yml && shx rm ./twbsconfig.yml",
-    "docs-github-serve": "bundle exec jekyll serve --skip-initial-build --no-watch",
+    "docs-github-serve": "npm run docs-serve -- --skip-initial-build --no-watch",
     "docs-lint": "npm-run-all docs-lint-*",
     "docs-lint-htmllint": "htmllint --rc build/.htmllintrc \"_gh_pages/**/*.html\" \"js/tests/**/*.html\"",
     "docs-lint-vnu-jar": "node build/vnu-jar.js",