goocanvas sample: use more specific name

Fix undefined method `prepend_environment_path'

ref: commit 510a0c216688d0209ecd5c8ec33e8eeaab3a07ee
Merge pull request #152 from myokoym/use-more-specific-name

goocanvas sample: use more specific name

Patch by Masafumi Yokoyama. Thanks!!!