Reenable anti-sudo check

[brson]

Disabled because it's being miscompiled. #698

[inejge]

Is there a known distro/nightly/rustup commit combination that produces a crashing rustup-init for 64-bit Linux? I tried with nightly-2016-08-21 and rustup pre-0.6.2 on CentOS 6, but couldn't get a segfault.

Edit: NVM, found the relevant issues. The interesting thing is that in my testing I don't get a segfault in every run. I thought I could rework the sudo check code to avoid the bug, but on further inspection it seems unlikely.

Does reenabling the check include reverting the changes to .travis.yml and appveyor.yml, or that should go into a separate PR?

[kinnison]

This looks to have been reenabled in e308c56