Unset the `ELECTRON_DISABLE_SECURITY_WARNINGS` environment variable
which was being used to disable the security checks when testing the app
on ci. This is no longer needed now that we have a more strict Content
Security Policy when the app runs in production mode.
See https://github.com/LN-Zap/zap-desktop/pull/456
grpc was being compiled twice when builds run on travis. This was causing
builds to take several minutes longer than they should. installation of
the dependencies listed in app/package.json happens automatically when
`electron-builder install-app-deps` runs as part of the postinstall step.
Tom Kirkpatrick
Jack Mallers