Add code coverage

.travis.yml

 language: go
+sudo: false
 go:
   - 1.12.x
   - 1.13.x
@@ -6,4 +7,7 @@ env:
   - PG_CONN=postgresql://postgres:@localhost:5432/postgres?sslmode=disable
 services:
   - postgresql
-scripts: go test github.com/casbin/casbin-pg-adapter
+before_install:
+  - go get github.com/mattn/goveralls
+script:
+  - $HOME/gopath/bin/goveralls -service=travis-ci

README.md

-[![Build Status](https://travis-ci.org/casbin/casbin-pg-adapter.svg?branch=master)](https://travis-ci.org/casbin/casbin-pg-adapter)
-
 # Go-pg Adapter
 
+[![Build Status](https://travis-ci.org/casbin/casbin-pg-adapter.svg?branch=master)](https://travis-ci.org/casbin/casbin-pg-adapter)
+[![Coverage Status](https://coveralls.io/repos/github/casbin/casbin-pg-adapter/badge.svg?branch=master)](https://coveralls.io/github/casbin/casbin-pg-adapter?branch=master)
+
 Go-pg Adapter is the [Go-pg](https://github.com/go-pg/pg) adapter for [Casbin](https://github.com/casbin/casbin). With this library, Casbin can load policy from PostgreSQL or save policy to it.
 
 ## Installation