Pin Lamina to fix the build.

Riemann 0.2.0 depends on Aleph 0.3.0-beta15, which depends on Lamina 0.5.0-beta14:

https://github.com/ztellman/aleph/commit/7216a660af8773e781e50ea548ef32de961ff0e4

Unfortunately, Lamina 0.5.0-beta14 depends on a nonexistent release of Coda Hale's Metrics library:

https://github.com/ztellman/lamina/blob/0.5.0-beta14/project.clj#L7

Lamina 0.5.0-beta15 no longer depends on Metrics at all, so the issue is resolved:

https://github.com/ztellman/lamina/blob/0.5.0-beta15/project.clj#L4

As soon as a version of Aleph is released that depends on Lamina >= 0.5.0-beta15, this line can be removed from the project.clj.