fixed travis build

Author: Sylvain MARIE

ci_tools/requirements-pip.txt

@@ -17,7 +17,7 @@ pytest-steps
 pytest-harvest
 
 # --- to generate the reports (see scripts in ci_tools, called by .travis)
-pytest-cov==$PYTEST_COV_VERSION  # after 2.6.1 it requires pytest 3.6
+pytest-cov$PYTEST_COV_VERSION  # after 2.6.1 it requires pytest 3.6
 
 # --- to generate the doc (see .travis)
 mkdocs-material  # ==2.7.3