Add dependencies to gemspec, version bump

Gemfile

 source 'https://rubygems.org'
-gem 'excon'
-gem 'json_pure'
-gem 'mime'
-
 gemspec

lib/packagecloud/version.rb

 module Packagecloud
   MAJOR_VERSION = "0"
   MINOR_VERSION = "2"
-  PATCH_VERSION = "16"
+  PATCH_VERSION = "17"
   VERSION = [MAJOR_VERSION, MINOR_VERSION, PATCH_VERSION].join(".")
 end

packagecloud-ruby.gemspec

@@ -17,6 +17,10 @@ Gem::Specification.new do |gem|
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
   gem.require_paths = ['lib']
+  gem.add_runtime_dependency 'excon', '~> 0.40'
+  gem.add_runtime_dependency 'json_pure', '~> 1'
+  gem.add_runtime_dependency 'mime', '~> 0.4'
+
   gem.add_development_dependency 'bundler', '~> 1.0'
   gem.add_development_dependency 'rake', '~> 0.8'
   gem.add_development_dependency 'rspec', '~> 2.4'