Stop installing rustfmt on CI

It's no longer present on every nightly so we're spuriously failing
builds

Author: Mark-Simulacrum

.travis.yml

@@ -4,9 +4,6 @@ sudo: true
 rust:
   - nightly
 
-before_script:
-  - rustup component add rustfmt-preview
-
 before_install:
   - sudo apt-get update
   - sudo apt-get install -y linux-tools-common linux-tools-generic linux-tools-`uname -r` linux-cloud-tools-`uname -r`