Build with panic=abort

[brson]

This will reduce binary size considerably.

cc #982

[brson]

This is a matter of changing .travil.yml and appveyor.yml.

[remram44]

I'm running into something similar to rust-lang/cargo#2738:

   Compiling rustup-mock v1.0.0 (file:///buildslave/src/rustup-mock)
error: the linked panic runtime `panic_unwind` is not compiled with this crate's panic strategy `abort`

Some dependencies need panic=unwind...

[brson]

One consideration with panic=abort: if an installation transaction panics then that transaction will not rollback. I'm not sure how heavily to weigh that - leaving unwinding on would certainly make rustup more resilient to bugs.