Fix #56 - decrease default keep-alive interval window for Heroku

As per Heroku http-routing docs[1]:
"After a dyno connection has been established, HTTP requests have an initial 30 second window in which the web process must return response data (either the completed response or some amount of response data to indicate that the process is active). Processes that do not send response data within the initial 30-second window will see an H12 error in their logs."

So to play better with Heroku timeouts, have decreaeed the default
Sente keep-alive intervals as follows:
`:ws-kalive-ms` - 38000 -> 25000
`:lp-kalive-ms` - 38000 -> 25000

https://devcenter.heroku.com/articles/http-routing#timeouts
v0.15.1