Run Puma in Single mode for development

Turns out how to switch between single and clustered modes of Puma is super unclear in the (little to non-existent) documentation. You'd think that setting `WEB_CONCURRENCY` to `1` would do it, but you actually have to set it to zero. Meaning you don't want to spin up any child processes.

obie-fernandez
May 11, 2017