It should fail releases with no start script present

The Node buildpack requires either a server.js script to be present or
for a start script to be specified in the package.json file. The
commit will cause the release phase of the buildpack to fail with a
suitable warning message if this is not the case.

[#71846414]
Merge branch 'cgfrost-71846414-fail_release_with_no_start_script'

[resolves #4]