change gemdir from "#{gem_clone_dir}/#{repo_name}" to "#{gem_clone_dir}/#{repo_owner}/#{repo_name}"

When building with external mrbgems, for example using these lines in build_config.rb:
```ruby
conf.gem :github => 'iij/mruby-digest'
``` 
The repo will be clone into *build/mrbgems/mruby-digest*. Each time I build, that path will be checked, created and pulled from remote if not exists.

Assume someday for some reason I want to use my own fork of that gem, and I updated build_config.rb to:
```ruby
conf.gem :github => 'dorentus/mruby-digest'
```
But when I run `./minirake all`, since the repo name is not changed and *build/mrbgems/mruby-digest* already exists, it does not got changed as expected. I have to remove *build/mrbgems/mruby-digest* manually to got it  updated.

----
This patch changes gemdir to "#{gem_clone_dir}/#{repo_owner}/#{repo_name}" and fixes problems above.