[travis] disable homebrew auto-update to speed up osx build

export HOMEBREW_NO_AUTO_UPDATE=1

https://github.com/Homebrew/brew/blob/master/Library/Homebrew/manpages/brew.1.md.erb#L194-L196

.travis.yml

@@ -23,7 +23,9 @@ matrix:
       env: TOXENV=py27-cov
     - language: generic
       os: osx
-      env: TOXENV=py36-cov
+      env:
+        - TOXENV=py36-cov
+        - HOMEBREW_NO_AUTO_UPDATE=1
 
 install:
   - ./.travis/install.sh

.travis/install.sh

@@ -15,7 +15,6 @@ if [ "$TRAVIS_OS_NAME" == "osx" ]; then
         python -m virtualenv .venv/
     elif [[ ${TOXENV} == *"py3"* ]]; then
         # install/upgrade current python3 with homebrew
-        brew update
         if brew list --versions python3 > /dev/null; then
             brew upgrade python3
         else