remove dup's in #update_gemfile for rspec generator

In `DaemonKit::Generators::SpecGenerator#update_gemfile` there is multiple entries for `group develoment, test` and `rspec`

```ruby
      def update_gemfile
        append_file 'Gemfile', "group :development, :test do\n  gem 'rspec'\nend\n"
        append_file 'Gemfile', <<-GEM
group :development, :test do
  gem 'rake'
  gem 'rspec'
end
GEM
      end
```

This produces duplication in `Gemfile` upon newly created daemon

```ruby
 group :development, :test do
    gem 'rspec'
 end
 group :development, :test do
    gem 'rake'
    gem 'rspec'
 end
```

Take care,
Brad