Bump a.js-int-tester version to ^3.1.0 (#24)

Author: Peripheral1994

package.json

@@ -29,7 +29,7 @@
   },
   "devDependencies": {
     "@segment/analytics.js-core": "^3.0.0",
-    "@segment/analytics.js-integration-tester": "^2.0.0",
+    "@segment/analytics.js-integration-tester": "^3.1.0",
     "@segment/clear-env": "^2.0.0",
     "@segment/eslint-config": "^3.1.1",
     "browserify": "^13.0.0",