Upgrade template to rspec3 style

After rspec3, rspec-rails generates model following like:

```
$ bundle exec rspec --version
3.0.2
$ bundle exec rails g model foo
      (snip)
      create      spec/models/foo_spec.rb
$ cat spec/models/foo_spec.rb
require 'rails_helper'

RSpec.describe Foo, :type => :model do
  pending "add some examples to (or delete) #{__FILE__}"
end
```

See also
https://github.com/rspec/rspec-rails/blob/master/lib/generators/rspec/model/templates/model_spec.rb