use travis yaml to install brew packages rather than installing manually

This is actually the recommended way of installing packages in Travis rather
than installing them manually in the build script. This helps get around being
caught by Ruby version issues (which we have recently).

See the following link for more information:
https://docs.travis-ci.com/user/installing-dependencies/#using-homebrew-without-addon-on-older-macos-images

.travis.yml

@@ -8,6 +8,15 @@ matrix:
     sudo: require
     dist: trusty
   - os: osx
+    addons:
+      homebrew:
+        packages:
+        - ffmpeg
+        - qt5
+        - grep
+        - opencolorio
+        - openimageio
+        update: true
   # Can't build on Windows - affected by https://travis-ci.community/t/current-known-issues-please-read-this-before-posting-a-new-topic/264/10
   # - os: windows
 

.travis/install.sh

@@ -2,8 +2,7 @@
 
 if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
 
-    brew install ffmpeg qt5 grep opencolorio openimageio
-    export PATH="/usr/local/opt/qt/bin:/usr/local/opt/python@2/libexec/bin:$PATH"
+    #export PATH="/usr/local/opt/qt/bin:/usr/local/opt/python@2/libexec/bin:$PATH"
 
 elif [[ "$TRAVIS_OS_NAME" == "linux" ]]; then