Rewrite from /bin/bash to pure /bin/sh

[SamuelMarks]

Advantages:

• Futureproofed
• No need for unfamiliar /bin/bash syntax for new users (they can just launch it in Bash, Zsh, Dash, or whatever else)
• Paves the way to port to new platforms (e.g., FreeBSD, busybox, MinGW, &etc.)

Also solves the weird bootstrap problem of /bin/bash <install brew> && brew install bash. Whether system Bash [or system Ruby for that matter] or a locally installed one is installed, in all cases it could be handled cleanly by using pure /bin/sh.

PS: Tell me if you want this and I'll finish the PR

[SMillerDev]

I don't think we support any platforms that don't ship some version of bash and I don't think it's a goal to support those either.
Seeing how much this complicates the already complicated script I'm not really in favour.

[MikeMcQuaid]

Passing on this, sorry.