Use Rack::Session::Cookie instead of enable :sessions
Use Rack::Session::Cookie instead of enable :sessions

When switching into device mode in Chrome, the session keys were
being cleared. But using Rack::Session::Cookie instead of
enable :sessions seems to fix this.

Resolves: #2492

copiousfreetime/exercism.io,treiff/exercism.io,nathanbwright/exercism.io,Tonkpils/exercism.io,mhelmetag/exercism.io,IanDCarroll/exercism.io,RaptorRCX/exercism.io,tejasbubane/exercism.io,RaptorRCX/exercism.io,Tonkpils/exercism.io,nathanbwright/exercism.io,copiousfreetime/exercism.io,jtigger/exercism.io,sheekap/exercism.io,mhelmetag/exercism.io,IanDCarroll/exercism.io,bmulvihill/exercism.io,sheekap/exercism.io,nathanbwright/exercism.io,copiousfreetime/exercism.io,tejasbubane/exercism.io,RaptorRCX/exercism.io,bmulvihill/exercism.io,treiff/exercism.io,IanDCarroll/exercism.io,k4rtik/exercism.io,mhelmetag/exercism.io,Tonkpils/exercism.io,IanDCarroll/exercism.io,k4rtik/exercism.io,k4rtik/exercism.io,tejasbubane/exercism.io,jtigger/exercism.io,jtigger/exercism.io,bmulvihill/exercism.io,treiff/exercism.io,treiff/exercism.io,bmulvihill/exercism.io,sheekap/exercism.io,Tonkpils/exercism.io,mhelmetag/exercism.io,copiousfreetime/exercism.io,jtigger/exercism.io,tejasbubane/exercism.io,sheekap/exercism.io,RaptorRCX/exercism.io,nathanbwright/exercism.io