Wait for pip's execution and actually check the return code
Wait for pip's execution and actually check the return code

When building the pip environment for the zip when using slim_handler, we're using `.poll()` which doesn't wait for the process to finish. `.wait()` would be the easier replacement, but it carries the risk of deadlocks, hence using `.communicate()`.
Miserlou/Zappa,scoates/Zappa,pjz/Zappa,Miserlou/Zappa,scoates/Zappa,pjz/Zappa