Merge pull request RustPython#420 from Tom1380/master

Uncommented a line of code like a TODO said.

Author: cthulahoops

tests/snippets/json_snippet.py

@@ -31,8 +31,7 @@ def round_trip_test(obj):
 assert 1 == json.loads("1")
 assert -1 == json.loads("-1")
 assert 1.0 == json.loads("1.0")
-# TODO: uncomment once negative floats are implemented
-# assert -1.0 == json.loads("-1.0")
+assert -1.0 == json.loads("-1.0")
 assert "str" == json.loads('"str"')
 assert True is json.loads('true')
 assert False is json.loads('false')