chore: fix Travis build: upgrade to node 12 (from 10)

.travis.yml

@@ -2,9 +2,8 @@ sudo: required
 dist: trusty
 language: node_js
 node_js:
-# - '6'
-# - '8'
-- '10'
+# - '10' # fails on Travis since upgrading to Vexflow 1.2.90 (still passes on AppVeyor), for Mxl_Test. Node 12 works.
+- '12'
 env:
   - timeout=10000
 notifications: