Specify version number for ujson and plac

The required version was specified for plac in requirements.txt but not in setup.py, which could cause a conflicting version error.
Similarly, set the version of ujson in requirements.txt to be the same as in setup.py

requirements.txt

@@ -7,6 +7,6 @@ thinc>=6.2.0,<6.3.0
 murmurhash>=0.26,<0.27
 plac<0.9.3
 six
-ujson
+ujson>=1.35
 cloudpickle
 sputnik>=0.9.2,<0.10.0

setup.py

@@ -234,7 +234,7 @@ def setup_package():
                 'cymem>=1.30,<1.32',
                 'preshed>=0.46.0,<0.47.0',
                 'thinc>=6.2.0,<6.3.0',
-                'plac',
+                'plac<0.9.3',
                 'six',
                 'cloudpickle',
                 'pathlib',