Use CommonJS tsconfig for testing

package.json

@@ -5,7 +5,7 @@
   "main": "build/index.js",
   "scripts": {
     "pretest": "rimraf ./build",
-    "test": "mocha -r ts-node/register -r tsconfig-paths/register \"src/**/*.ts\"",
+    "test": "TS_NODE_PROJECT=\"tsconfig.spec.json\" mocha -r ts-node/register -r tsconfig-paths/register \"src/**/*.ts\"",
     "test.coverage": "nyc npm test",
     "test.watch": "npm test -- --watch",
     "prepublishOnly": "npm run build --production",

tsconfig.spec.json

@@ -0,0 +1,7 @@
+{
+    "extends": "./tsconfig.json",
+    "compilerOptions": {
+        "module": "commonjs",
+        "target": "es6"
+    }
+}