Fixing requirements versions to avoid issues

Coverage on last builds were NOT working due to some change on Python
coverage package

Author: suoto

requirements.txt

@@ -1,12 +1,19 @@
-nose2
-mock
-nose2-cov
-argcomplete
-prettytable
-testfixtures
-coverage
-codecov
-coveralls
-colorama
-logutils
-vunit_hdl
+argcomplete==1.4.1
+codecov==2.0.5
+colorama==0.3.7
+cov-core==1.15.0
+coverage==4.1
+coveralls==1.1
+docopt==0.6.2
+funcsigs==1.0.2
+logutils==0.3.3
+mock==2.0.0
+nose2==0.6.5
+nose2-cov==1.0a4
+pbr==1.10.0
+prettytable==0.7.2
+rainbow-logging-handler==2.2.2
+requests==2.10.0
+six==1.10.0
+testfixtures==4.10.0
+vunit-hdl==0.66.0