chore(deps): Upgrade less to v4 (prerelease) (shaka-project#4028)

In shaka-project#3991, I changed the syntax of our colors to a modern rgba syntax.
For example, rgba(255, 255, 255, 0.85) would become rgba(255 255 255 /
85%). However, less v3 seems not to understand that properly, and
performs division on the last two parts, resulting in output of
rgba(255 255 3%), which is indeed invalid.

This fixes the issue by upgrading to less v4, which understands the
new rgba syntax and leaves it alone. The output for that will now
match the input.

To work around an issue with less v4, this uses a prerelease version
with a fix for less/less.js#3693 . See also
tomas/needle#391

This doesn't affect any release branches, since shaka-project#3991 hasn't been
cherry-picked.

Closes shaka-project#4027

package.json

@@ -45,7 +45,7 @@
     "karma-sourcemap-loader": "^0.3.8",
     "karma-spec-reporter": "^0.0.33",
     "karma-webdriver-launcher": "^1.0.8",
-    "less": "^3.13.1",
+    "less": "https://gitpkg.now.sh/joeyparrish/less.js/packages/less?28c63a43",
     "less-plugin-clean-css": "github:austingardner/less-plugin-clean-css#4e9e77bf",
     "material-design-lite": "^1.3.0",
     "mux.js": "^5.14.1",