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
Merge pull request #13 from deg/master

Give better error message on failure of exec of external process.
fix accidental double word
fix spelling typos with lein-spell
Merge pull request #16 from cldwalker/fix-typos

Fix typos
0.1.1 changelog and readme update
0.1.1
0.1.2-SNAPSHOT
Merge pull request #1 from cemerick/master

Catch up with upstream from cemerick/austin