Travis CI was failing while testing with Go 1.10 because the latest testify requires
Go 1.13 for error.As. So by adding go.mod to use a specific testify version (and testing
with Go 1.11 minimally since we now need 'go mod') the tests should compile.
Travis CI was failing while testing with Go 1.10 because the latest testify requires Go 1.13 for error.As. So by adding go.mod to use a specific testify version (and testing with Go 1.11 minimally since we now need 'go mod') the tests should compile.