fixed: TypeError in pkg._find_install_targets when version is None

here's an example traceback:

    State: - pkg
    Name:      libgeos-c1
    Function:  installed
         Result:    False
         Comment:   An exception occurred in this state: Traceback (most recent call last):
    File "/usr/lib/pymodules/python2.7/salt/state.py", line 1201, in call
     *cdata['args'], **cdata['kwargs'])
    File "/usr/lib/pymodules/python2.7/salt/states/pkg.py", line 325, in installed
     result = _find_install_targets(name, version, pkgs, sources)
    File "/usr/lib/pymodules/python2.7/salt/states/pkg.py", line 95, in _find_install_targets
     if version in cver:
    TypeError: 'in <string>' requires string as left operand, not NoneType