(optim/ci): cache Electron binary to speed up builds

- saw this on some blog posts, but caching on CI isn't directly
  documented in Electron for some reason
  - caching is mentioned in the installation docs though
    - https://github.com/electron/electron/blob/master/docs/tutorial/installation.md#cache

Author: agilgur5

.travis.yml

@@ -2,5 +2,10 @@ language: node_js
 # default is apparently 0.10.48 for some reason O.O
 node_js: '10.16.0'
 
+# cache folder with Electron binary (https://github.com/electron/electron/blob/master/docs/tutorial/installation.md#cache)
+cache:
+  directories:
+    - ~/.cache
+
 # run with xvfb bc it's needed for Electron (https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-the-xvfb-run-wrapper)
 script: xvfb-run npm test