Require pygobject 3 now that it was ported to gi

Commit 540e2e460c8155a22b10c1712eec71c9bf25aea9 ported to
gobject introspection but this never reflected in configure.ac.
Fix summary for invest applet
Merge pull request #70 from infirit/master

Require pygobject 3 now that it was ported to gi