increase required 'requests' version to 2.11.0

- uses an OrderedDict for session.headers (since 2.9.2)
- ships with urllib3 1.16, which is the first version to have an
  'allowed_gai_family()' function

requirements.txt

@@ -1 +1 @@
-requests
+requests>=2.11.0

setup.py

@@ -102,7 +102,7 @@ setup(
     license="GPLv2",
     python_requires=">=3.4",
     install_requires=[
-        "requests >= 2.4.2",
+        "requests>=2.11.0",
     ],
     packages=[
         "gallery_dl",