Use SSL for connecting to Postgres

This is needed by Heroku
(https://devcenter.heroku.com/articles/heroku-postgresql#heroku-postgres-ssl),
but KnexJS doesn't have support for it. We needed to do a small hack to make it
work while knex/knex#852 isn't fixed.

knexfile.js

@@ -2,6 +2,11 @@
 
 require('dotenv').config();
 
+// FIXME: Needed because Heroku requires SSL-only connections and Knex doesn't
+// allow setting it (see https://github.com/tgriesser/knex/issues/852)
+const pg = require('pg');
+pg.defaults.ssl = true;
+
 const db = {
   production: {
     client: 'pg',