Rack cnb also supports the bionic stack (#5)

Signed-off-by: Arjun Sreedharan <asreedharan@vmware.com>

Author: Arjun Sreedharan

buildpack.toml

@@ -4,9 +4,12 @@ api = "0.2"
   id = "paketo-community/rackup"
   name = "Rackup Buildpack"
 
-[[stacks]]
-  id = "org.cloudfoundry.stacks.cflinuxfs3"
-
 [metadata]
   include_files = ["bin/run", "bin/build","bin/detect","buildpack.toml"]
   pre_package = "./scripts/build.sh"
+
+[[stacks]]
+  id = "io.buildpacks.stacks.bionic"
+
+[[stacks]]
+  id = "org.cloudfoundry.stacks.cflinuxfs3"