repl-env: Add precondition to ensure :session-id is string

Using a numeric session ID (e.g. 123) produces a URL that looks correct
(e.g. http://localhost:48240/123/repl/start), but when used to start a REPL
prints the error "Austin ClojureScript REPL session 123 does not exist."
Merge pull request #8 from noidi/ensure-session-id-is-string

repl-env: Add precondition to ensure provided :session-id is string