`epix_slide` won't grab the latest version

[kenmawer]

Related to #146, but the ref_time_values, which should actually be ref_versions, does not take in the latest version for the slide.

[brookslogan]

From call: we should just fix the latest version not being included here, and leave the renaming for a separate discussion. #146 contained some ideas for setting a default.

[brookslogan]

Closed by #311.