diff --git a/Makefile b/Makefile
index f5dfaaa29..d81384bfd 100644
--- a/Makefile
+++ b/Makefile
@@ -173,7 +173,7 @@ mypy: ${PYSOURCES}
 	MYPYPATH=typeshed/2.7 mypy --py2 --disallow-untyped-calls schema_salad
 
 jenkins:
-	if ! test -d env ; then virtualenv env ; fi
+	rm -Rf env && virtualenv env
 	. env/bin/activate ; \
 	pip install -U setuptools pip wheel ; \
 	${MAKE} install-dep coverage.html coverage.xml pep257_report.txt \
diff --git a/requirements.txt b/requirements.txt
index 8c64938e7..aa195c893 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 requests
-ruamel.yaml
-rdflib >= 4.1.
-rdflib-jsonld >= 0.3.0
+ruamel.yaml==0.11.11
+rdflib>=4.1.
+rdflib-jsonld>=0.3.0
 mistune
 typing ; python_version>="2.7"
 avro ; python_version<"3"