You can stream it, but it's a pain and you have to have use a log in for a service that already has the ATT Sports Channel or whatever they changed it to from Roots. I use my parents Direct TV log in.
You have to download the MLB At Bat App. Make the Astros your favorite team. When a game starts open the app and scroll down a little on the Astros page and you will see an orange icon that says "watch live". Click on that and it will have you enter your Direct TV log in and then the game will play. Airplay it up on the TV and you are good to go.
And no you do not have to pay for MLB At Bat.
Here is as long thread about how to do it if my instructions didn't cut it.
http://bbs.clutchfans.net/index.php?threads/finally-root-sports-will-allow-live-online-streaming.280461/