Fix setup import error

`import gspread` statement caused requests packaged to be imported before
dependency could be installed. Sole purpose of import was to extract the
__version__ string field on the module.

Solution is to use a regex to read the version string from __init__.py

Fixes burnash#317

setup.py

@@ -1,10 +1,9 @@
 #!/usr/bin/env python
 
 import os.path
+import re
 import sys
 
-import gspread
-
 try:
     from setuptools import setup
 except ImportError:
@@ -32,12 +31,15 @@ def read(filename):
 
 long_description = long_description.lstrip("\n").format(index=read('docs/index.txt'))
 
+version = re.search(r'^__version__\s*=\s*[\'"]([^\'"]*)[\'"]',
+                    read('gspread/__init__.py'), re.MULTILINE).group(1)
+
 setup(
     name='gspread',
     packages=['gspread'],
     description=description,
     long_description=long_description,
-    version=gspread.__version__,
+    version=version,
     author='Anton Burnashev',
     author_email='[email protected]',
     url='https://github.com/burnash/gspread',