Update the Play application build

The existing build depends on the Play cli being installed in order to
build the test Play application. The Play cli has been deprecated and
replaced with the Typesafe activator cli. This commit updates the
build and readme to use the new Typesafe activator.
Merge branch 'typesafe-activator'

[resolves #11]
Test applications support for request headers

The suite of test applications for the java buildpack do not support an end
point for displaying the HTTP request headers recieved by the application.
This is a useful feature and this commit adds support.

[#76709518]