Update requirements (#4125)

requirements/requirements-codestyle.txt

@@ -3,4 +3,4 @@ flake8==2.4.0
 pep8==1.5.7
 
 # Sort and lint imports
-isort==4.2.2
+isort==4.2.5

requirements/requirements-documentation.txt

 # MkDocs to build our documentation.
-mkdocs==0.13.2
+mkdocs==0.15.3

requirements/requirements-optionals.txt

 # Optional packages which may be used with REST framework.
-markdown==2.6.4
-django-guardian==1.3.2
+markdown==2.6.6
+django-guardian==1.4.4
 django-filter==0.10.0

requirements/requirements-packaging.txt

 # Wheel for PyPI installs.
-wheel==0.24.0
+wheel==0.29.0
 
 # Twine for secured PyPI uploads.
-twine==1.4.0
+twine==1.6.5
 
 # Transifex client for managing translation resources.
 transifex-client==0.11

requirements/requirements-testing.txt

 # PyTest for running the tests.
-pytest==2.8.5
+pytest==2.9.1
 pytest-django==2.9.1
 pytest-cov==1.8.1