update shebang to reflect bashisms

[mcandre]

Update shebang, given that local is not a POSIX semantic. Discovered this when trying to run the rustup installer on Oracle Solaris, where /bin/sh is a symlink to ksh93.

Note that the rustup.rs website should also pipe to bash, not sh.

Alternatively, we can rewrite this script in pure POSIX sh, though I leave that to cooler heads.

[mcandre]

Looks like one of the macOS CI builds stalled for this tweak, dunno why?

https://travis-ci.org/rust-lang-nursery/rustup.rs/jobs/290991285

[mati865]

If changing to bash using #!/usr/bin/env bash should be preferred.

[mcandre]

@mati865 Eh, if the shell were ash, dash, or posh, or relied on alpha features from Homebrew bash tip, then /usr/bin/env would definitely help to port the script to varied systems. But for common shell languages like bash, sh, ksh, zsh, the /usr/bin/env prefix just slows things down unnecessarily.

[Diggsey]

Thanks for the PR!