Update gemspec to fix warnings

- Specifying `licenses` is recommended
- More helpful `summary` and `description`

gemoji.gemspec

@@ -1,14 +1,15 @@
 Gem::Specification.new do |s|
   s.name    = "gemoji"
   s.version = "1.5.0"
-  s.summary = "Emoji Assets"
-  s.description = "Emoji assets"
+  s.summary = "Emoji conversion and image assets"
+  s.description = "Image assets and character information for emoji."
 
   s.required_ruby_version = '~> 1.9'
 
   s.authors  = ["GitHub"]
   s.email    = "support@github.com"
   s.homepage = "https://github.com/github/gemoji"
+  s.licenses = ["MIT"]
 
   s.files = Dir[
     "README.md",