Update dependencies in requirements_dev.txt (#188)

The following updates were done by pyup-bot:

- Update pytest from 3.6.3 to 3.6.4 (#184)
- Update pip from 10.0.1 to 18.0 (#178)
- Update cryptography from 2.2.2 to 2.3 (#173)
- Update sphinx from 1.7.5 to 1.7.6 (#166)
- Update tox from 3.0.0 to 3.1.2 (#162)

requirements_dev.txt

@@ -1,18 +1,18 @@
-pip==10.0.1
+pip==18.0
 bumpversion==0.5.3
 wheel==0.31.1
 watchdog==0.8.3
 flake8==3.5.0
 mock==2.0.0
-tox==3.0.0
+tox==3.1.2
 coverage==4.5.1
-Sphinx==1.7.5
-cryptography==2.2.2
+Sphinx==1.7.6
+cryptography==2.3
 PyYAML==3.13 # pyup: ignore
 future==0.16.0
 scipy>=0.17.0
 numpy>=1.13.0
 matplotlib>=1.3.1
-pytest==3.6.3
+pytest==3.6.4
 attrs==18.1.0
 pre-commit