Fix UnicodeDecodeError

```python
Traceback (most recent call last):
  File "install_erpnext.py", line 348, in <module>
    install()
  File "install_erpnext.py", line 13, in install
    install_pre_requisites()
  File "install_erpnext.py", line 25, in install_pre_requisites
    install_using_yum()
  File "install_erpnext.py", line 90, in install_using_yum
    update_config_for_redhat()
  File "install_erpnext.py", line 106, in update_config_for_redhat
    exec_in_shell("chkconfig --level 2345 %s on" % service)
  File "install_erpnext.py", line 339, in exec_in_shell
    if err and any((kw in err.lower() for kw in ["traceback", "error", "exception"])):
  File "install_erpnext.py", line 339, in <genexpr>
    if err and any((kw in err.lower() for kw in ["traceback", "error", "exception"])):
```
Merge pull request #971 from MaxMorais/patch-1

Fix UnicodeDecodeError