Add code coverage to CI

[zemyblue]

I commented out the code coverage test in CircleCI in the last commit (ref #10).
But it is necessary to know which code is tested or not throght this test

So we need to add code coverage test again.

[zemyblue]

We cannot use codecov, because this repository is private and our account(line) already use one free private repository supports.

We need to find alternative service.

[zemyblue]

I didn't find alternative service to fit to us.
coveralls is also private is not free.

I think this should be added after it becomes an opensource. 😭
I'll close this issue.