chore(travis): ensure npm version

Author: christianalfoni

.travis.yml

@@ -17,6 +17,7 @@ git:
   depth: 9999
 notifications:
   email: false
+before_install: dpkg --compare-versions `npm -v` ge 6.3 || npm i -g npm@^6.3
 script:
 - |
   if [[ $SKIP_TESTS != 'true' ]]; then