overwrite: Encode output to utf-8 to prevent error

I needed to make this change to prevent the following error when running `py.test`:

```
INTERNALERROR>   File "/Users/marca/dev/git-repos/pytest-sugar/pytest_sugar.py", line 264, in pytest_runtest_logreport
INTERNALERROR>     self.overwrite(self.insert_progress())
INTERNALERROR>   File "/Users/marca/dev/git-repos/pytest-sugar/pytest_sugar.py", line 251, in overwrite
INTERNALERROR>     sys.stdout.write("\r" + self.append_string(line))
INTERNALERROR> UnicodeEncodeError: 'ascii' codec can't encode character u'\u2713' in position 52: ordinal not in range(128)
```

The problem is that Python's default encoding, AFAIK, is ASCII, so if you try to write a non-ASCII Unicode character (in this case, U+2713, the checkmark), you get a `UnicodeEncodeError`.

I guess that you didn't see this error on your system? Which is curious. I wonder if you have configured Python or your terminal to use UTF-8 so that masked the problem?

In any case, I think this change will make pytest-sugar work on a wider variety of systems.