Use `nanshe/nanshe_notebook:latest` on Wercker
Use `nanshe/nanshe_notebook:latest` on Wercker

Docker assumes that the `latest` tag of an image is desired if no tag is
specified. Instead of leaving this up to Wercker's Docker to guess,
explicitly specify the version `nanshe/nanshe_notebook` to use as
`latest`.

nanshe-org/nanshe_workflow,DudLab/nanshe_workflow