Travis-CI: Fix nvm failing to install Node.js v12.4.0

Something must have changed on the travis-ci end to cause things to suddenly fail
2020-04-06 11:05:45 +01:00 · 2020-04-06 11:05:45 +01:00 · c259aea3bc
commit c259aea3bc
--- a/.travis.yml
+++ b/.travis.yml
@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - "12.4.0"
+  - "12"

 stages:
  - name: test