add go.mod

Author: PeterIvanov

.travis.yml

@@ -1,9 +1,10 @@
 language: go
 
-before_script: cd $GOPATH/src/github.com/stretchr/testify/ && git checkout v1.2.2 && cd -
+before_script: [[ $TRAVIS_GO_VERSION =~ (^1\.8) ]]; then cd $GOPATH/src/github.com/stretchr/testify/ && git checkout v1.2.2 && cd -; fi
 
 go:
   - "1.8.x"
+  - "1.11.x"
   - "1.12.x"
   - "1.13.x"
   - "1.14.x"