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

- per Cypress Travis CI directions
  - https://docs.cypress.io/guides/guides/continuous-integration.html#Travis
  - the other stuff isn't necessary for our specific setup or
    are currently Travis CI defaults
    - like install: npm ci and cache: npm: true are defaults

Author: agilgur5

.travis.yml

@@ -1,3 +1,8 @@
 language: node_js
 # default is apparently 0.10.48 for some reason O.O
 node_js: '10.16.0'
+
+# cache folder with Cypress binary (https://docs.cypress.io/guides/guides/continuous-integration.html#Travis)
+cache:
+  directories:
+    - ~/.cache