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."

I got bitten by this while trying to fix the URL to a constant value (my HTTP server runs in a different process so I can't easily pass the generated URL to it). The problem took me quite a while to track down. This precondition should prevent others from making the same mistake.