Fix stable BWC branch detection logic
Fix stable BWC branch detection logic

This commit fixes the logic for finding the stable BWC branch. A .x
branch should only be chosen if we are testing BWC with a previous major
version.

kalimatas/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,fred84/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,qwerty4030/elasticsearch,rajanm/elasticsearch,rajanm/elasticsearch,maddin2016/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,scottsom/elasticsearch,robin13/elasticsearch,ThiagoGarciaAlves/elasticsearch,masaruh/elasticsearch,HonzaKral/elasticsearch,s1monw/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,qwerty4030/elasticsearch,robin13/elasticsearch,wangtuo/elasticsearch,scottsom/elasticsearch,masaruh/elasticsearch,kalimatas/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,ThiagoGarciaAlves/elasticsearch,ThiagoGarciaAlves/elasticsearch,GlenRSmith/elasticsearch,qwerty4030/elasticsearch,HonzaKral/elasticsearch,scorpionvicky/elasticsearch,s1monw/elasticsearch,robin13/elasticsearch,gfyoung/elasticsearch,ThiagoGarciaAlves/elasticsearch,coding0011/elasticsearch,wenpos/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,scottsom/elasticsearch,scottsom/elasticsearch,rajanm/elasticsearch,masaruh/elasticsearch,ThiagoGarciaAlves/elasticsearch,scorpionvicky/elasticsearch,rajanm/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,wangtuo/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,wenpos/elasticsearch,kalimatas/elasticsearch,wenpos/elasticsearch,maddin2016/elasticsearch,maddin2016/elasticsearch,s1monw/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,masaruh/elasticsearch,maddin2016/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,wangtuo/elasticsearch,wangtuo/elasticsearch,wenpos/elasticsearch,rajanm/elasticsearch,uschindler/elasticsearch,wenpos/elasticsearch,GlenRSmith/elasticsearch,rajanm/elasticsearch,qwerty4030/elasticsearch,scottsom/elasticsearch,scorpionvicky/elasticsearch,fred84/elasticsearch,s1monw/elasticsearch,kalimatas/elasticsearch,fred84/elasticsearch,coding0011/elasticsearch,wangtuo/elasticsearch,ThiagoGarciaAlves/elasticsearch,GlenRSmith/elasticsearch,kalimatas/elasticsearch,s1monw/elasticsearch,qwerty4030/elasticsearch,fred84/elasticsearch,maddin2016/elasticsearch,masaruh/elasticsearch,gfyoung/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,fred84/elasticsearch,uschindler/elasticsearch