Install script touch ups

[danielpclark]

@baptistecolin I've tried this script on a clean Docker image and it doesn't get past rustc.

docker run -i -t ubuntu /bin/bash
apt-get update
apt-get install curl
curl https://raw.githubusercontent.com/danielpclark/abrute/master/install.sh | bash

Output:

bash: line 3: rustc: command not found
bash: line 16: [: ==: unary operator expected

[danielpclark]

Maybe using which rustc to see if that produces a value would be better?

[danielpclark]

Would it be okay with you if I assign this to you? I can label it Hacktoberfest for you if you're trying to get the free shirt.

[baptistecolin]

oops, I have to fix that indeed.
I will take a look at this shortly.

[baptistecolin]

I made a PR to fix this : #10