Update demo service to be compatible with AWS bundler

The lambci docker image updated their bundler version to match AWS,
so we need to follow suit.

lambci/docker-lambda@5b48a7c#diff-f44c2756eda09ae41c9bc9ff6956aa31

Author: joshuaflanagan

README.md

@@ -137,3 +137,14 @@ yarn test               # run the automated test suite
 
 ./integration_test.sh   # run serverless package on the demo project and observe the results
 ```
+
+### Troubleshooting the demo_service
+
+* Make sure the version of bundler specified in the demo_service Gemfile.lock
+is compatible with the version installed in the lambci/lambda:build-ruby2.5 image.
+
+* Make sure you can invoke the function n in the docker image:
+
+```
+./invoke-service.sh
+```

__tests__/demo_service/Gemfile.lock

@@ -38,4 +38,4 @@ DEPENDENCIES
   rspec-core
 
 BUNDLED WITH
-   1.17.3
+   2.1.4

dev_setup.sh

@@ -1,5 +1,5 @@
 yarn
 pushd __tests__/demo_service
-bundle --standalone --path vendor/bundle
+./install-gems.sh
 yarn
 popd

integration-test.sh

@@ -6,4 +6,4 @@ echo '### 1) EXAMINE PACKAGE'
 unzip -l .serverless/demo.zip
 echo
 echo '### 2) VERIFY FUNCTION CAN LOAD DEPENDENCIES'
-serverless invoke local -f hello
+./invoke-service.sh