fix(travis): set DEBUG=electron-builder on travis

Enable electron-builder debugging on travis to ensure that output is
produced whilst long running electron (re)build steps are running. This
can prevent build timeouts.

.travis.yml

@@ -3,6 +3,9 @@ sudo: false
 language: node_js
 
 env:
+  global:
+    - DEBUG=electron-builder
+  matrix:
   - TEST=lint-ci
   - TEST=test-ci