[close #88] Raise error when no source given

[schneems]

  14) Java a simple java app on jdk-openjdk-9.0.4 should deploy
      Failure/Error:
        Hatchet::Runner.new(
          buildpacks: ["https://github.com/heroku/heroku-buildpack-java"],
          config: {
            "JVM_COMMON_BUILDPACK" => "https://api.github.com/repos/heroku/heroku-buildpack-jvm-common/tarball/#{jvm_common_branch}"
          }
        ).tap do |app|
          # app.before_deploy do
          #   set_java_version(Dir.pwd, version)
          # end
          # app.deploy do

      ArgumentError:
        unknown keywords: :buildpacks, :config
      # /Users/rschneeman/.gem/ruby/2.7.1/gems/heroku_hatchet-6.0.0/lib/hatchet/config.rb:49:in `[]'
      # /Users/rschneeman/.gem/ruby/2.7.1/gems/heroku_hatchet-6.0.0/lib/hatchet/config.rb:49:in `block in path_for_name'
      # /Users/rschneeman/.gem/ruby/2.7.1/gems/heroku_hatchet-6.0.0/lib/hatchet/config.rb:48:in `each'
      # /Users/rschneeman/.gem/ruby/2.7.1/gems/heroku_hatchet-6.0.0/lib/hatchet/config.rb:48:in `detect'
      # /Users/rschneeman/.gem/ruby/2.7.1/gems/heroku_hatchet-6.0.0/lib/hatchet/config.rb:48:in `path_for_name'
      # /Users/rschneeman/.gem/ruby/2.7.1/gems/heroku_hatchet-6.0.0/lib/hatchet/app.rb:40:in `initialize'
      # ./test/spec/java_spec.rb:17:in `new'
      # ./test/spec/java_spec.rb:17:in `block (4 levels) in <top (required)>'

This PR defaults to a value then raises an error when that value is not assigned manually by the programmer.

[edmorley]

@schneems Hi! I don't know if you'd seen it, but there was an intermittent on the CI of this post merge:
https://circleci.com/gh/heroku/hatchet/709

[schneems]

Thanks, I logged it in my "flappy tests" issue. CI seems to be less robust than "regular" tests.