Fix Linux & MacOS install from release instructions

[alexbalonperin]

I tried using the provided instructions to download terraformer from release on PopOS but the grep on tag_name wasn't returning anything so I have adjusted the instructions. I suggest one solution with jq which is the most elegant/robust imo but requires an extra dependency and one solution using vanilla grep for those who don't want to bother installing jq.

Hopefully that helps other people get terraformer installed.

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[sergeylanzman]

@alexbalonperin can you to sign a Contributor License Agreement (CLA)?