unpin requirements

I'd like to propose not strictly pinning dependencies in this package, allowing users of this library to have their own pinned versions instead. If accepted and released, fantastic! If rejected, I'll make a fork (for now) that allows us to relax the dependencies.

setup.py

@@ -56,16 +56,16 @@
         'Topic :: Database :: Front-Ends',
     ],
     install_requires=[
-        'click==6.7',
-        'future==0.16.0',
-        'ipaddress==1.0.14',
-        'requests==2.13.0',
-        'requests_kerberos==0.10.0',
-        'six==1.10.0',
-        'typing==3.5.3.0',
+        'click',
+        'future',
+        'ipaddress',
+        'requests',
+        'requests_kerberos',
+        'six',
+        'typing',
     ],
     extras_require={'tests':[
-        'httpretty==0.8.14',
+        'httpretty',
         'pytest',
         'pytest-runner',
     ]}