Require rdflib_jsonld for development.

requirements_dev.txt

@@ -15,6 +15,7 @@ numpy>=1.15.1
 pytest-flask
 pytest>=3.2.2
 rdflib==4.2.2
+rdflib_jsonld==0.4.0
 requests-toolbelt>=0.8.0
 requests>=2.18.4
 sphinx-rtd-theme>=0.2.4