Simplify Mac builds on Travis

- don't specify a fixed build environment version, use the default instead
  (https://docs.travis-ci.com/user/reference/osx#OS-X-Version)
- hide the `brew update` output unless there is an error
  (this is 1500+ lines in recent builds)
- remove an error message related to installing libpng, which is already
  a dependency of python3

Author: jkseppan

.travis.yml

@@ -85,7 +85,6 @@ matrix:
     - python: "nightly"
       env: PRE=--pre
     - os: osx
-      osx_image: xcode7.3
       language: generic  # https://github.com/travis-ci/travis-ci/issues/2312
       env: MOCK=mock
       only: master
@@ -109,8 +108,8 @@ before_install:
       export PATH=$PATH:/tmp/λ
       export PATH=/usr/lib/ccache:$PATH
     else
-      brew update
-      brew install python3 libpng ffmpeg imagemagick mplayer ccache
+      ci/travis/silence brew update
+      brew install python3 ffmpeg imagemagick mplayer ccache
       # make 'python' mean 'python3'
       ln -sf /usr/local/bin/python3 /usr/local/bin/python
       hash -r

ci/travis/silence

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+# Run a command, hiding its standard output and error if its exit
+# status is zero.
+
+stdout=$(mktemp -t stdout) || exit 1
+stderr=$(mktemp -t stderr) || exit 1
+"$@" >$stdout 2>$stderr
+code=$?
+if [[ $code != 0 ]]; then
+    cat $stdout
+    cat $stderr >&2
+    exit $code
+fi