Electron-Builder 20.0+ defaults linux builds to Snap instead of AppImage

[andrew-c-tran]

What happened (or feature request):

As of electron-builder 20.0, the default linux build target is Snap where it was previously defaulted to AppImage in v19.33.0. This is causing an unexpected build failure when building the binaries. Note that this is a silent failure and results in passed circle tests even though linux did not build!

In the interim, package-lock.json will have electron-builder require 19.33.0 until we get this sorted.

Steps needed:

• Override default snap target to use AppImage
• Ensure build correctly builds all 3 OS binaries in newest version

[dclear]

Focusing on Mac for v0.4.1 alpha. Will pick up with next release.