Update to Durus 3.9

Update to Durus 2.8 release.

Merge #64 to trunk:

`durus.connection`
 - Remove side-effect from assert statement.
 
`durus`
 - Bump version to 3.7-schevo2
 
SchevoDurus
 - require Durus 3.7-schevo2
 
Schevo
 - rely on SchevoDurus >= 3.1a1dev-r3527, which in turn relies on Durus 3.7-schevo2