update social media icon links

[kgjenkins]

The icons at bottom of pages need updates:

• remove google plus (inactive)
• github should link to qgis-us repo

[kgjenkins]

Fixed by https://github.com/qgis-us/qgis-us.github.io/pull/49/commits/df82d1ea74d477a8b90e2a8de1f35c3cf13393a4