Fix an issue of exposed coveralls repo_token

According to [Coveralls docs](https://coveralls.io/docs/ruby), it seems that the option `repo_token` should be kept secret:
>  The option repo_token ...  should be kept secret -- anyone could use it to submit coverage data on your repo's behalf.

I fixed the issue and the result is seemed to be success:  
[ishida/itunes-client | Build #1 | Coveralls](https://coveralls.io/builds/78021)