installation commands with yarn

contribute.py

@@ -69,8 +69,8 @@ def main(date, name):
     # launch the presentation deck
     if not os.path.exists(os.path.join(fullPath, 'node_modules')):
         click.echo(' > Installing dependencies ... ')
-        os.system('npm install -g npm@latest grunt-cli generator-reveal')
-        os.system('npm install')
+        os.system('yarn add -g grunt-cli generator-reveal')
+        os.system('yarn')
         click.echo(' > All dependencies installed.')
     else:
         click.echo(' > All dependencies already installed.')