Build an export file in CI, too

[joshwlewis]

Without an export file, subsequent testpacks can't use Ruby.

This example repo will not compile on CI or pass tests when using the heroku/ruby buildpack. The example repo expects bundle and ruby to be available on the $PATH during the node buildpack bin/test-compile and bin/test stages. With this change, the example repo builds and passes tests on CI.

[schneems]

This has been deployed