Fix typo in requirements.

requirements.txt

@@ -9,4 +9,4 @@ pyjwt>=1.5.3,<1.6
 iso8601>=0.1.12,<0.2
 arrow>=0.10.0,<0.11
 bleach>=2.1.1,<2.2
-snapshottest=>0.5.0,<0.6
+snapshottest>=0.5.0,<0.6