Recognize CocoaPods documents as Ruby source files.

Examples:
* https://github.com/alloy/cocoapods/blob/master/examples/MacRubySample/Podfile
* https://github.com/alloy/cocoapods-specs/blob/master/SSToolkit/0.1.2/SSToolkit.podspec

Also sorted filenames in languages.yml.

lib/linguist/languages.yml

@@ -817,6 +817,7 @@ Ruby:
   - .gemspec
   - .god
   - .irbrc
+  - .podspec
   - .rake
   - .rb
   - .rbuild
@@ -827,10 +828,11 @@ Ruby:
   - .watchr
   filenames:
   - Capfile
-  - Rakefile
-  - Thorfile
   - Gemfile
   - Guardfile
+  - Podfile
+  - Rakefile
+  - Thorfile
   - Vagrantfile
 
 Rust: