fix(package): update knex to version 0.20.15

Closes #95

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "http-status": "~1.3.2",
     "joi": "~14.3.1",
     "jsonade": "~1.4.1",
-    "knex": "~0.16.3",
+    "knex": "~0.20.15",
     "lodash": "~4.17.11",
     "mandrill-api": "~1.0.45",
     "pg": "~7.9.0",