Use current version for all libraries.

Author: todoa2c

requirements.txt

@@ -1,9 +1,9 @@
-cov-core==1.7
-coverage==3.7
-execnet==1.1
-py==1.4.17
-pytest==2.4.2
-pytest-cov==1.6
-pytest-xdist==1.9
+cov-core==1.15.0
+coverage==4.4.1
+execnet==1.4.1
+py==1.4.34
+pytest==3.1.3
+pytest-cov==2.5.1
+pytest-xdist==1.18.0
 tox==2.7.0
 virtualenv==15.1.0