Remove Bountysource from Readme

Bountysource appears to be dead, so it should be removed

README.md

@@ -1,6 +1,6 @@
 # Natron
 
-[![GPL2 License](http://img.shields.io/:license-gpl2-blue.svg?)](https://github.com/NatronGitHub/Natron/blob/master/LICENSE.txt) [![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v1.4%20adopted-ff69b4.svg)](CODE_OF_CONDUCT.md) [![Build Status](https://api.travis-ci.org/NatronGitHub/Natron.svg?branch=RB-2.4)](https://travis-ci.org/NatronGitHub/Natron) [![Coverage Status](https://coveralls.io/repos/NatronGitHub/Natron/badge.svg?branch=master)](https://coveralls.io/r/NatronGitHub/Natron?branch=master) [![Documentation Status](https://readthedocs.org/projects/natron/badge/?version=rb-2.4)](http://natron.readthedocs.io/en/rb-2.4/) [![Packaging status](https://repology.org/badge/tiny-repos/natron.svg)](https://repology.org/project/natron/badges) [![BountySource Status](https://api.bountysource.com/badge/team?team_id=271309)](https://www.bountysource.com/teams/natrongithub/issues?utm_source=Mozilla&utm_medium=shield&utm_campaign=bounties_received) [![OpenHub](https://www.openhub.net/p/natron/widgets/project_thin_badge?format=gif&ref=Thin+badge)](https://www.openhub.net/p/Natron)
+[![GPL2 License](http://img.shields.io/:license-gpl2-blue.svg?)](https://github.com/NatronGitHub/Natron/blob/master/LICENSE.txt) [![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v1.4%20adopted-ff69b4.svg)](CODE_OF_CONDUCT.md) [![Build Status](https://api.travis-ci.org/NatronGitHub/Natron.svg?branch=RB-2.4)](https://travis-ci.org/NatronGitHub/Natron) [![Coverage Status](https://coveralls.io/repos/NatronGitHub/Natron/badge.svg?branch=master)](https://coveralls.io/r/NatronGitHub/Natron?branch=master) [![Documentation Status](https://readthedocs.org/projects/natron/badge/?version=rb-2.4)](http://natron.readthedocs.io/en/rb-2.4/) [![Packaging status](https://repology.org/badge/tiny-repos/natron.svg)](https://repology.org/project/natron/badges) [![OpenHub](https://www.openhub.net/p/natron/widgets/project_thin_badge?format=gif&ref=Thin+badge)](https://www.openhub.net/p/Natron)
 
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