Initial Update (#524)

* Pin pycodestyle to latest version 2.4.0

* Pin pytest to latest version 3.5.0

* Pin yapf to latest version 0.21.0

* Update requirements.txt

Author: pyup-bot

tests/requirements.txt

@@ -1,9 +1,9 @@
 keras>=2.1,<2.2
-pycodestyle<2.4.0,>=2.0.0
+pycodestyle>=2.4,<2.5
 pydocstyle>=2.1,<2.2
-pytest>=3.4,<3.5
+pytest>=3.5,<3.6
 pytest-cache>=1.0,<1.1
 pytest-cov>=2.5,<2.6
 pytest-xdist>=1.22,<1.23
 sphinx>=1.7,<1.8
-yapf>=0.20,<0.21
+yapf>=0.21,<0.22