Here's what you need:
(Install it normally by tapping the cloud icon saying Version 87 and then tap on Install)
1. Install the latest 'stable' version of the Google App.
(In my case, the version is 220.127.116.11arm64 on my m3note)2. Download the xposed module, Assistant Enabler from the Xposed Installer itself.Though it says, its for Marshmallow ROMs but it works on Lollipop too.
3. Enable the module and restart your device.
4. Open the Google app and complete the onscreen instructions for setting up the app. Now say OK Google and Voila! You got the Google Assistant!
Credits to the author of that guide, click here to check it.