add missing package to travisci

Author: maxmousee

.travis.yml

@@ -2,6 +2,7 @@ language: go
 install:
   - go get golang.org/x/tools/cmd/cover
   - go get github.com/mattn/goveralls
+  - go get github.com/stretchr/testify
 script:
   - go test -v -covermode=count -coverprofile=coverage.out
   - 'if [ "$TRAVIS_PULL_REQUEST" = "false" ] && [ ! -z "$COVERALLS_TOKEN" ]; then $HOME/gopath/bin/goveralls -coverprofile=coverage.out -service=travis-ci -repotoken $COVERALLS_TOKEN; fi'