specify version requirements for urllib3

urllib3 versions 1.24.1 and 1.24.2 cause HTTP requests to
https://www.artstation.com/users/<username>/quick.json
to fail with a 403: Forbidden status code (#227),
and provoke a CAPTCHA response after solving a Cloudflare challenge.

requirements.txt

@@ -1 +1,2 @@
 requests>=2.11.0
+urllib3>=1.16, !=1.24.1, !=1.24.2

setup.py

@@ -103,6 +103,7 @@ setup(
     python_requires=">=3.4",
     install_requires=[
         "requests>=2.11.0",
+        "urllib3>=1.16, !=1.24.1, !=1.24.2",
     ],
     packages=[
         "gallery_dl",