Change duration requirement of ad from 39 seconds to 40, fixes #10

The culprit of [issue 10](https://github.com/ArtemGordinsky/SpotiFree/issues/10) is [an advertisement exactly 40 seconds in length](https://www.evernote.com/shard/s116/sh/cfc09595-ae7c-44ac-9944-38d3b8cd8adf/27a13013b3c37b7f6fceb8df020403b2). I changed the condition from `< 40` to `<= 40` and the advertisement now mutes.