builder: strip ansi color code of build log

Before:
![image](https://cloud.githubusercontent.com/assets/2300438/5156663/75e7fdd6-731a-11e4-900c-bfe353d49fdb.png)

After:
![image](https://cloud.githubusercontent.com/assets/2300438/5156666/94752206-731a-11e4-8b89-1d6176177f4d.png)

Actual terminal output:

execute below command:
```
python -c "print('\n'.join([(' '.join([('tr((i + j)) + 'm' + str((i + j)).ljust(5) + '') if i + j < 256 else '' for j in range(10)])) for i in range(0, 256, 10)]))"
```

![image](https://cloud.githubusercontent.com/assets/2300438/5156665/8affb3d0-731a-11e4-97ec-7da868cce1e1.png)