travis: add shellcheck
travis: add shellcheck

sebrandon1/etcd,armstrongli/etcd,vimalk78/etcd,gyuho/etcd,ajityagaty/etcd,joshix/etcd,visheshnp/etcd,mqliang/etcd,li-ang/etcd,ajityagaty/etcd,sinsharat/etcd,heyitsanthony/etcd,fanminshi/etcd,mqliang/etcd,rnaveiras/etcd,visheshnp/etcd,mkumatag/etcd,armstrongli/etcd,bluebreezecf/etcd,li-ang/etcd,mkumatag/etcd,mqliang/etcd,bluebreezecf/etcd,coreos/etcd,cockroachdb/etcd,fanminshi/etcd,xiang90/etcd,dannysauer/etcd,heyitsanthony/etcd,sebrandon1/etcd,rnaveiras/etcd,akauppi/etcd,lorneli/etcd,glevand/coreos--etcd,a-robinson/etcd,bdarnell/etcd,lorneli/etcd,akauppi/etcd,xiang90/etcd,etcd-io/etcd,mickep76/etcd,YuleiXiao/etcd,vimalk78/etcd,dannysauer/etcd,gyuho/etcd,joshix/etcd,whuwxl/etcd,whuwxl/etcd,mkumatag/etcd,sinsharat/etcd,bdarnell/etcd,cockroachdb/etcd,coreos/etcd,rnaveiras/etcd,mickep76/etcd,ajityagaty/etcd,glevand/coreos--etcd,YuleiXiao/etcd,etcd-io/etcd,a-robinson/etcd,etcd-io/etcd