fix(travis): Don't call brew update in Travis CI

Brew disabled updating from shallow clones:
https://github.com/Homebrew/brew/pull/9383

The update is also uneeded since our .travis.yml already has `update: true` for
the brew extension.

.travis/cirp/install.sh

@@ -5,8 +5,6 @@
 # Get Python >=3.5
 if [ "$TRAVIS_OS_NAME" == "osx" ]
 then
-  brew update
-
   # make sha256sum available
   export PATH="/usr/local/opt/coreutils/libexec/gnubin:$PATH"