Fixed issue for installing on Mac OS when JAVA_HOME variable is not set

I got following error when running on Mac OS without JAVA_HOME variable:
```
Traceback (most recent call last):
  File "setup.py", line 54, in <module>
    platform_specific['library_dir'] = [os.path.join(java_home, 'Libraries')]
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'
```
So I just move `platform_specific['library_dir']` to assign only of java_home is defined