Remove developer environment-specific ignores

Gitignores should be releated to the project, not individual developers' set up. The latter should be in a global gitignore.
Some of these appear to be "standard" for rubygem projects:

- https://github.com/github/gitignore/blob/master/Ruby.gitignore#L1-2
- https://github.com/github/gitignore/blob/master/Ruby.gitignore#L6
- https://github.com/github/gitignore/blob/master/Ruby.gitignore#L27-31