Add codecov coverage threshold

Author: nfisher

codecov.yml

@@ -1,10 +1,21 @@
 ignore:
   - index_gen.go
+
+coverage:
+  status:
+    project:
+      default:
+        target: 55%
+        threshold: 5%
+        base: auto
+        flags:
+          - unit
+
 comment:
   layout: "reach, diff, flags, files"
   behavior: default
   require_changes: false  # if true: only post the comment if coverage changes
   require_base: no        # [yes :: must have a base report to post]
-  require_head: yes       # [yes :: must have a head report to post]
+  require_head: no       # [yes :: must have a head report to post]
   branches:               # branch names that can post comment
     - "master"