Upgraded some dev dependencies

To sync up with Pillar dependency versions. Also marked coverage as secondary dependency (because it is).

requirements-dev.txt

@@ -1,17 +1,17 @@
--e .
+-e ../pillar-python-sdk
 
 # Development requirements
 tox==2.3.1
 wheel==0.29.0
-coverage==4.0.3
-pytest==3.0.1
+pytest==3.0.6
 pytest-xdist==1.14
-pytest-cov==2.3.1
+pytest-cov==2.4.0
 responses==0.5.1
 
 # Secondary development requirements
 apipkg==1.4
 cookies==2.2.1
+coverage==4.3.4
 execnet==1.4.1
 pluggy==0.3.1
 py==1.4.32