Update to work with latest briefcase version for OSX builds.

.travis.yml

@@ -40,7 +40,7 @@ install:
   # Install Mu dependencies
   - pip install -r requirements.txt
   # Install packager for OSX
-  - pip install briefcase==0.2.4
+  - pip install briefcase
   # Install disk image maker for OSX
   - pip install dmgbuild 
   # Install helpful pytest plug-in
@@ -58,7 +58,7 @@ script:
   # Package it for OSX
   #- if [ "$TRAVIS_OS_NAME" = "osx" ]; then echo "[install]" >> setup.cfg; echo "prefix=" >> setup.cfg; fi
   #- if [ "$TRAVIS_OS_NAME" = "osx" ]; then sudo mkdir -p /lib/python3.6/site-packages/; fi
-  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then python setup.py macos --support-pkg=https://github.com/mu-editor/mu_portable_python_macos/releases/download/0.0.5/python3-reduced.tar.gz; fi
+  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then python setup.py macos --support-pkg=https://github.com/mu-editor/mu_portable_python_macos/releases/download/0.0.6/python3-reduced.tar.gz; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then mkdir dist; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then zip -r -X dist/mu-editor.zip macOS/mu-editor.app; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then du -sk dist/; fi