[webapp] use puma web server instead of rackup

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
[webapp] limit number of puma threads to one - webapp is not thread safe

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
Merge pull request #10 from sailfish-sdk/puma

Use puma webserver