Update favicon.png

Causes has a new logo and a new favicon. This commit replaces the old
favicon, which was a green square with a white "C" superimposed, with
the new favicon, which is a circle filled with polygons of color.

Change-Id: If30d4db665e6601f2d84fa45dadd7c3199329f12
Reviewed-on: https://gerrit.causes.com/29890
Tested-by: Joe Lencioni <joe.lencioni@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>