resetting tracked sessions in KillAndWait, to add more preditablity while running on travis

gexec/session.go

diff --git a/gexec/session.go b/gexec/session.go
index 3cbfded..6d6ad34 100644
--- a/gexec/session.go
+++ b/gexec/session.go
@@ -229,6 +229,7 @@
 	for _, session := range trackedSessions {
 		session.Kill().Wait(timeout...)
 	}
+	trackedSessions = []*Session{}
 }
 
 /*