Jekyll expects gems as an array

Looks like more recent (?) versions of Jekyll expect `gems:` to be an array.

Using the previously specified syntax I get:

    jekyll 3.0.1 | Error:  undefined method `each' for "-octopress-image-tag":String

Author: dgmstuart

README.md

@@ -22,8 +22,7 @@ Then install the gem with Bundler
 
 Then add the gem to your Jekyll configuration.
 
-    gems:
-      -octopress-image-tag
+    gems: [octopress-image-tag]
 
 ## Usage