Fix coverage script for jenkins build

On OS X 'tail +2 <foo>' works, but not on linux

cover.sh

diff --git a/cover.sh b/cover.sh
index 59dc9b4..eea15cc 100755
--- a/cover.sh
+++ b/cover.sh
@@ -44,7 +44,7 @@
 for d in $(go list ./... | grep -v vendor); do
     go test -coverprofile=profile.out -covermode=$coverMode $d
     if [ -f profile.out ]; then
-        tail +2 profile.out >> coverage.txt
+        tail -n+2 profile.out >> coverage.txt
         rm profile.out
     fi
 done