Use yarn instead of npm to fetch node dependencies

Fixes #5

tasks.py

@@ -116,7 +116,7 @@ def assets(ctx, watch=False):
 
     # Be sure to have all the dependencies installed
     if not os.path.exists(node_bin):
-        invoke.run("npm install", pty=True)
+        invoke.run("yarn", pty=True)
 
     if watch:
         try: