fix cover.sh to maske it compatible with goobuntu

cover.sh

diff --git a/cover.sh b/cover.sh
index 7c312e5..a378f75 100755
--- a/cover.sh
+++ b/cover.sh
@@ -6,7 +6,7 @@
 for d in $(go list ./... | grep -v vendor); do
     go test -coverprofile=profile.out -covermode=atomic $d
     if [ -f profile.out ]; then
-        tail +2 profile.out >> coverage.txt
+        tail -n +2 profile.out >> coverage.txt
         rm profile.out
     fi
 done