support to load migrations from the classpath

hello,

this pull request adds support to load migrations from the classpath. I'm not sure if you want this to be part of ragtime.sql.files or if you will prefer to add loading migrations from the classpath in the new module.

I'm be happy to create a new module if you thing that's a better approach.

Sorry about having to bump up the clojure version to 1.5.1. I'm using Lighttable, which requires clojure 1.5.