[FIX] partner_firstname: Mixed content error, move to HTTPS

Fix https://github.com/OCA/partner-contact/pull/487#issuecomment-396026511

partner_firstname/README.rst

@@ -1,5 +1,5 @@
 .. image:: https://img.shields.io/badge/licence-AGPL--3-blue.svg
-    :target: http://www.gnu.org/licenses/agpl-3.0-standalone.html
+    :target: https://www.gnu.org/licenses/agpl-3.0-standalone.html
     :alt: License: AGPL-3
 
 ================================
@@ -105,9 +105,9 @@ Translations
 Maintainer
 ----------
 
-.. image:: http://odoo-community.org/logo.png
+.. image:: https://odoo-community.org/logo.png
    :alt: Odoo Community Association
-   :target: http://odoo-community.org
+   :target: https://odoo-community.org
 
 This module is maintained by the OCA.
 
@@ -115,4 +115,4 @@ OCA, or the Odoo Community Association, is a nonprofit organization whose
 mission is to support the collaborative development of Odoo features and
 promote its widespread use.
 
-To contribute to this module, please visit http://odoo-community.org.
+To contribute to this module, please visit https://odoo-community.org.