Add a direct dependency on ansi-colors

It's used directly in src/index.js. Without this, if I used
plugin-lambda-invoker in a project that has no other transitive
dependencies on ansi-colors, I get:

```
Error: Plugin error:
- Unable to load plugin 'architect/plugin-lambda-invoker': Cannot find module 'ansi-colors'
```

package.json

@@ -25,6 +25,7 @@
   "homepage": "https://github.com/architect/plugin-lambda-invoker#readme",
   "dependencies": {
     "@architect/utils": "^3.1.2",
+    "ansi-colors": "^4.1.1",
     "enquirer": "^2.3.6"
   },
   "devDependencies": {