fix rbenv module UnboundLocalError

This commit fixes the error of the rbenv module below :

```
2014-07-23 15:48:42,068 [salt.state       ][ERROR   ] An exception occurred in this state: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1517, in call
    **cdata['kwargs'])
  File "/usr/lib/python2.7/dist-packages/salt/states/rbenv.py", line 162, in installed
    return _check_and_install_ruby(ret, name, default, user=user)
  File "/usr/lib/python2.7/dist-packages/salt/states/rbenv.py", line 83, in _check_and_install_ruby
    if __salt__['rbenv.install_ruby'](ruby, runas=user):
  File "/usr/lib/python2.7/dist-packages/salt/modules/rbenv.py", line 181, in install_ruby
    if build_env:
UnboundLocalError: local variable 'build_env' referenced before assignment
```