use abstract dependency instead of concrete ones

Author: tifayuki

requirements.txt

@@ -1,4 +1,3 @@
-backports.ssl-match-hostname==3.4.0.2
 future==0.15.0
 requests==2.7.0
 six==1.9.0

setup.py

@@ -4,6 +4,12 @@
 
 from setuptools import setup, find_packages
 
+requirements =[
+    "future >= 0.15.0, < 1",
+    "requests >= 2.5.2, < 3",
+    "six >= 1.3.0, < 2",
+    "websocket-client >= 0.32.0, < 1"
+]
 
 def read(*parts):
     path = os.path.join(os.path.dirname(__file__), *parts)
@@ -20,14 +26,11 @@ def find_version(*file_paths):
     raise RuntimeError('Unable to find version string.')
 
 
-with open('requirements.txt') as f:
-    install_requires = f.read().splitlines()
-
 setup(
         name="python-dockercloud",
         version=find_version('dockercloud', '__init__.py'),
         packages=find_packages(),
-        install_requires=install_requires,
+        install_requires=requirements,
         provides=['docker'],
         include_package_data=True,
         author="Docker, Inc.",