travis: Switch to Node 10, from Node 8.
travis: Switch to Node 10, from Node 8.

This is the current LTS, and what I've been using for some time
and what we recommend in our setup instructions in build-run.md .
Hadn't noticed that our Travis config was still on the older version.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile