Requirement: update orjson

We had it pinned because it made Travis to fail, but we are on CircleCI now.
Let's see if we can make it work with the latest version of it.

We need to update it because `cargo` is failing to install this package due that
it doesn't find some dependencies.

Author: humitos

requirements/pip.txt

@@ -57,8 +57,8 @@ pyquery==1.4.3
 # NOTE: this dep can be removed in python 3.7 in favor of ``date.fromisoformat``
 python-dateutil==2.8.2
 
-# Ignoring orjson for now because it makes Travis to fail
-orjson==2.0.7  # pyup: ignore
+# Version 3.6.3 is not available for Python 3.6
+orjson==3.6.1  # pyup: ignore
 
 # Utils
 django-gravatar2==1.4.4