Add deadLetter as a valid function

We want to upgrade to a newer serverless version, to do that, we need to
make this change to work with the new serverless dsl.

This was taken from the original repo which is no longer maintained and
the proposed fix was not merged.

gmetzker#44

Author: ivantoniev

src/index.js

@@ -5,6 +5,11 @@ const BbPromise = require('bluebird');
 class Plugin {
 
   constructor(serverless, options) {
+    serverless.configSchemaHandler.defineFunctionProperties('aws', {
+      properties: {
+        deadLetter: { type: 'object' }
+      }
+    });
 
     this.serverless = serverless;
     this.options = options;

tests/index.test.js

@@ -52,7 +52,10 @@ describe('serverless-plugin-lambda-dead-letter', () => {
 
       },
       cli: { log: () => {
-      } }
+      } },
+      configSchemaHandler: {
+        defineFunctionProperties: () => {}
+      }
     };
 
     return serverless;
@@ -97,7 +100,8 @@ describe('serverless-plugin-lambda-dead-letter', () => {
 
       // ARRANGE:
       const serverlessStub = {
-        getProvider: sinon.stub()
+        getProvider: sinon.stub(),
+        configSchemaHandler: { defineFunctionProperties: () => {} },
       };
       const provider = { kother: 'v' };
       const options = { k: 'v' };