Scheduled monthly dependency update for June (#25)

* Update mock from 2.0.0 to 3.0.5

* Update pip from 19.1 to 19.1.1

* Update pytest from 4.4.1 to 4.5.0

* Update pytest-runner from 4.4 to 5.1

* Update tox from 3.9.0 to 3.12.1

* Update wheel from 0.33.1 to 0.33.4

requirements_dev.txt

@@ -1,13 +1,13 @@
 bump2version==0.5.10
 coverage==4.5.3
 flake8==3.7.7
-mock==2.0.0
-pip==19.1
-pytest==4.4.1
-pytest-runner==4.4
+mock==3.0.5
+pip==19.1.1
+pytest==4.5.0
+pytest-runner==5.1
 six==1.12.0
 Sphinx==1.8.5 # pyup: <2.0 # for Python 2.7 and 3.4 support
-tox==3.9.0
+tox==3.12.1
 twine==1.13.0
 watchdog==0.9.0
-wheel==0.33.1
+wheel==0.33.4