Add dalli adapter gemspec

Just having the file exist in the flipper gem is odd so let's instead have a gem where we can define the dependencies and versions required.

flipper-dalli.gemspec

@@ -0,0 +1,24 @@
+# -*- encoding: utf-8 -*-
+require File.expand_path('../lib/flipper/version', __FILE__)
+
+flipper_dalli_files = lambda { |file|
+  file =~ /dalli/
+}
+
+Gem::Specification.new do |gem|
+  gem.authors       = ["John Nunemaker"]
+  gem.email         = ["nunemaker@gmail.com"]
+  gem.summary       = "Dalli adapter for Flipper"
+  gem.description   = "Dalli adapter for Flipper"
+  gem.license       = "MIT"
+  gem.homepage      = "https://github.com/jnunemaker/flipper"
+
+  gem.files         = `git ls-files`.split("\n").select(&flipper_dalli_files) + ["lib/flipper/version.rb"]
+  gem.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n").select(&flipper_dalli_files)
+  gem.name          = "flipper-dalli"
+  gem.require_paths = ["lib"]
+  gem.version       = Flipper::VERSION
+
+  gem.add_dependency 'flipper', "~> #{Flipper::VERSION}"
+  gem.add_dependency 'dalli', '>= 2.0', '< 3'
+end

lib/flipper-dalli.rb

@@ -0,0 +1 @@
+require 'flipper/adapters/dalli'