Downgrade terser to 3.14

This was done to avoid running into [this
issue](https://github.com/vuejs/vue-cli/issues/3407)
3 jobs for master in 8 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #932
build:lektor

00:06:08

 
  Staging
passed #933
staging:testing

00:02:47

 
  Deploy
manual #934
allowed to fail manual
deploy:production