Dont list activemodel as dev- and runtime dep

activemodel is a runtime dependency, it doesn't need to be listed as
development dependency as well.

hcloud.gemspec

@@ -23,7 +23,6 @@ Gem::Specification.new do |spec|
  spec.executables = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
  spec.require_paths = ['lib']
 
- spec.add_development_dependency 'activemodel'
  spec.add_development_dependency 'bundler'
  spec.add_development_dependency 'grape'
  spec.add_development_dependency 'rspec'