reslove WARNINGs in gem build

WARNING:  licenses is empty, but is recommended.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
WARNING:  open-ended dependency on rspec (>= 1.2.9, development) is not recommended
  if rspec is semantically versioned, use:
    add_development_dependency 'rspec', '~> 1.2', '>= 1.2.9'
WARNING:  open-ended dependency on json_pure (>= 1.2.0) is not recommended
  if json_pure is semantically versioned, use:
    add_runtime_dependency 'json_pure', '~> 1.2', '>= 1.2.0'
WARNING:  open-ended dependency on hashie (>= 0.2.0) is not recommended
  if hashie is semantically versioned, use:
    add_runtime_dependency 'hashie', '~> 0.2', '>= 0.2.0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: tropo-webapi-sdk-ruby
  Version: 15.11.0
  File: tropo-webapi-sdk-ruby-15.11.0.gem
XIANGJYU-M-21FJ:tropo-webapi-ruby xiangjyu$ gem build tropo-webapi-ruby.gemspec
  Successfully built RubyGem
  Name: tropo-webapi-sdk-ruby
  Version: 15.11.0
  File: tropo-webapi-sdk-ruby-15.11.0.gem

tropo-webapi-ruby.gemspec

@@ -6,6 +6,7 @@
 Gem::Specification.new do |s|
   s.name = %q{tropo-webapi-sdk-ruby}
   s.version = "15.11.0"
+  s.licenses = ['MIT']
 
   s.required_rubygems_version = Gem::Requirement.new(">= 0") if s.respond_to? :required_rubygems_version=
   s.authors = ["Frank Yu"]
@@ -41,9 +42,9 @@ Gem::Specification.new do |s|
     s.specification_version = 3
 
     if Gem::Version.new(Gem::VERSION) >= Gem::Version.new('1.2.0') then
-      s.add_development_dependency(%q<rspec>, [">= 1.2.9"])
-      s.add_runtime_dependency(%q<json_pure>, [">= 1.2.0"])
-      s.add_runtime_dependency(%q<hashie>, [">= 0.2.0"])
+      s.add_development_dependency 'rspec', '~> 1.2', '>= 1.2.9'
+      s.add_runtime_dependency 'json_pure', '~> 1.2', '>= 1.2.0'
+      s.add_runtime_dependency 'hashie', '~> 0.2', '>= 0.2.0'
     else
       s.add_dependency(%q<rspec>, [">= 1.2.9"])
       s.add_dependency(%q<json_pure>, [">= 1.2.0"])