Specify minimum versions for dependencies

docs/troubleshooting.rst

@@ -9,8 +9,3 @@ Troubleshooting
   supply it as a parameter to ``verify`` in your config file, e.g.::
 
       verify = /usr/share/ca-certificates/cacert.org/cacert.org_root.crt
-
-- **During sync an error occurs: TypeError: request() got an unexpected keyword
-  argument 'verify'**
-
-  You need to update your version of requests.

setup.py

@@ -39,8 +39,8 @@ setup(
     },
     install_requires=[
         'click>=3.1',
-        'requests',
-        'lxml',
+        'requests>=2.1',
+        'lxml>=3.0',
         'icalendar>=3.6',
         'requests_toolbelt>=0.3.0'
     ],