expand jsonschema deps as well

Author: jarobe42

setup.py

@@ -10,8 +10,8 @@
       classifiers=['Programming Language :: Python :: 3 :: Only'],
       url="http://singer.io",
       install_requires=[
-          'pytz>=2018.4',
-          'jsonschema==2.6.0',
+          'pytz>=2018.4,<=2019.3',
+          'jsonschema>=2.6.0,<=3.2.0',
           'simplejson==3.11.1',
           'python-dateutil>=2.6.0',
           'backoff==1.8.0',