requirements.txt: Update setuptools

Updates setuptools to version 21 to resolve runtime errors
on certain platforms.

(cherry-picked from d56cc7e)

.travis.yml

@@ -37,7 +37,11 @@ before_install:
     fi
 
   # Use pip 9, so that setuptools can be downgraded.
-  - pip install pip==9 setuptools==17
+  # https://github.com/coala/coala/issues/4708
+  # Apart from builds with python 2.7 and 3.5, it installs setuptools
+  # twice. So, it is required to uninstall one version manually.
+  - pip uninstall setuptools --yes
+  - pip install pip==9 setuptools==21
   - python .misc/check_setuptools.py
 
   # https://github.com/coala/coala/issues/3183

requirements.txt

@@ -7,5 +7,5 @@ libclang-py3~=3.4.0
 Pygments~=2.1
 PyPrint~=0.2.6
 requests~=2.12
-setuptools>=17.0
+setuptools>=21.0.0
 unidiff~=0.5.2