Returns the status codes in the report responses

[andrysds]

for issue #9

[nitishm]

@andrysds Can you fix the failures reported in Travis.

$ make validate
golangci-lint run
internal/vegeta/report.go:131:16: Error return value of `json.Unmarshal` is not checked (errcheck)
    json.Unmarshal(buf.Bytes(), &m)
                  ^
internal/vegeta/report.go:135:16: Error return value of `json.Unmarshal` is not checked (errcheck)
    json.Unmarshal(osd, &oldStatusCodes)
                  ^
internal/vegeta/report.go:148:29: Error return value of `(*encoding/json.Encoder).Encode` is not checked (errcheck)
    json.NewEncoder(buf).Encode(m)
                               ^
make: *** [validate] Error 1
The command "make validate" exited with 2.

[nitishm]

Great job @andrysds !! Thanks ! 🎉

[andrysds]

Thanks @nitishm !! 🎉