Remove nonexistent .rake file from gemspec

Fixes warning while bundling gems:
```
acts_as_commentable at /home/sergey/.rvm/gems/ruby-2.0.0-p0/bundler/gems/acts_as_commentable-    a637277ce9b4 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["tasks/acts_as_commentable_tasks.rake"] are not files
```