Use File.expand_path correct
Remove guard clause
Use File.join
Remove guard clause
Fix descriptions
Rename install_hook to install

This is a code review from @leafac:
https://github.com/stupied4ever/ruby-git-hooks/pull/4#discussion_r15411800
Change `git_hooks configure`to `git_hooks init`
Use hash fetch with default instead `|| default`
Fix typo
Remove assertion on doesn't raise

@leafac review:
https://github.com/stupied4ever/ruby-git-hooks/pull/4#discussion_r15413009
Use fixture instead of stub YML
Avoid mocking what I don't own