travis: remove node v5, add node latest (#993)
travis: remove node v5, add node latest (#993)

Node v5 is no longer supported by Polymer or even the Node team. Lets change travis to test LTS & the latest version as per our support policy.
StartPolymer/polymer-starter-kit-plus,StartPolymer/polymer-starter-kit-plus,abdonrd/polymer-starter-kit,abdonrd/polymer-starter-kit