Raising an error when response returns any status code but 200.

This fixes #10
While I really like `TravisPy.github_auth` raising an error, raising an exception for `job.build` and `job.log` feels weird for me. Seems that it may lead to code like this:

```python
try:
    build = job.build
except TravisError:
    # not sure what the user can do in this case
else:
    # job has a build    
```

Can you give an example in which real-life situation a job wouldn't have a associated `build` or `log`? I haven't used the travis api extensively, so some enlightment here is appreciated! :smile: