compatibility patch for python 3.4

[udaykrishna]

fixed the following issues

• mkdir's exist_ok parameter was introduced in python 3.5 so it won't work

  TypeError                                 Traceback (most recent call last)
  <ipython-input-51-e68d872ac2ee> in <module>()
  ----> 1 pdfreader = PDFBox()
  
  <ipython-input-50-102fb593b824> in __init__(self)
      113 
      114     def __init__(self):
  --> 115         self.pdfbox_path = self._get_pdfbox_path()
      116         self.java_path = shutil.which('java')
      117         if not self.java_path:
  
  <ipython-input-50-102fb593b824> in _get_pdfbox_path(self)
      95                 raise RuntimeError('error retrieving %s' % pdfbox_url)
      96             else:
  ---> 97                 cache_dir.mkdir(exist_ok=True, parents=True)
      98                 pdfbox_path = cache_dir.joinpath(pathlib.Path(pdfbox_url).name)
      99                 with open(pdfbox_path, 'wb') as f:
  
  TypeError: mkdir() got an unexpected keyword argument 'exist_ok'

• creating a file with Posix path on returns TypeError: invalid file:

  TypeError                                 Traceback (most recent call last)
  <ipython-input-53-e68d872ac2ee> in <module>()
  ----> 1 pdfreader = PDFBox()
  
  <ipython-input-52-f0e3aef44255> in __init__(self)
      114 
      115     def __init__(self):
  --> 116         self.pdfbox_path = self._get_pdfbox_path()
      117         self.java_path = shutil.which('java')
      118         if not self.java_path:
  
  <ipython-input-52-f0e3aef44255> in _get_pdfbox_path(self)
      98                     cache_dir.mkdir(parents=True)
      99                 pdfbox_path = cache_dir.joinpath(pathlib.Path(pdfbox_url).name)
  --> 100                 with open(pdfbox_path, 'wb') as f:
      101                     f.write(data)
      102 
  
  TypeError: invalid file: PosixPath('/home/ubuntu/.cache/python-pdfbox/pdfbox-app-2.0.14.jar')

Tested on python 3.4 in ubuntu 14.04