Change the go.image subrepo import path

Based on
https://groups.google.com/forum/#!msg/golang-nuts/eD8dh3T9yyA/l5Ail-xfMiAJ
all of the code.google.com/p/go.* subrepos are getting moved to
golang.org/x/* this commit points the
`code.google.com/p/go.image/tiff` to `golang.org/x/image`