Modify AutoUpdater behavior on macOS.

The AutoUpdater requires that macOS builds be code signed. That is currently not possible.
As a workaround, the autoupdater on mac will now alert users that an update is available and give the option to download the file directly from GitHub.
Closes #16.

.travis.yml

@@ -8,6 +8,7 @@ matrix:
         - ELECTRON_CACHE=$HOME/.cache/electron
         - ELECTRON_BUILDER_CACHE=$HOME/.cache/electron-builder
         - ELECTRON_BUILDER_ALLOW_UNRESOLVED_DEPENDENCIES=true
+        - CSC_IDENTITY_AUTO_DISCOVERY=false
 
     - os: linux
       services: docker