ci(release): 3.1.2 (#972)

Version bump in package.json and package-lock.json for release 3.1.2

Attempting to do this manually since something seems to be broken with
the automation (duffel-bot is not creating the PRs). Will look into that
next.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@duffel/api",
-  "version": "3.1.1",
+  "version": "3.1.2",
   "description": "Javascript client library for the Duffel API",
   "main": "dist/index.js",
   "module": "dist/index.es.js",