Upgrade dependencies (#59)

* Upgrade dependencies

- There was a security vuln with mocha, so upgraded mocha.
- Upgraded versions of node we're going to check in travis
- Switched to yarn from npm
- Removed --no-exit as that's standard mocha behavior now

* Enable postgres on newer version of travis

Author: brianc

Diff for: .travis.yml

@@ -1,8 +1,14 @@
 language: node_js
+dist: trusty
 node_js:
-  - "4"
-  - "6"
   - "8"
   - "10"
+  - "12"
 env:
-  - PGUSER=postgres PGDATABASE=postgres
+  - PGUSER=postgres
+services:
+  - postgresql
+addons:
+  postgresql: "9.6"
+before_script:
+  - psql -c 'create database travis;' -U postgres | true