Highlight Ruby method definitions without arguments

Inheriting from `clike` is enough to deal with constructs like this

```ruby
def foo(bar)
  ...
end
```

but not

```ruby
def foo
  ...
end
```

This does not highlight call sites, but since much idiomatic Ruby
doesn't use parentheses in function calls, call sites can't be
consistently highlighted anyway.

components/prism-ruby.js

@@ -13,7 +13,12 @@
 				greedy: true
 			}
 		],
-		'keyword': /\b(?:alias|and|BEGIN|begin|break|case|class|def|define_method|defined|do|each|else|elsif|END|end|ensure|false|for|if|in|module|new|next|nil|not|or|protected|private|public|raise|redo|require|rescue|retry|return|self|super|then|throw|true|undef|unless|until|when|while|yield)\b/
+		'keyword': /\b(?:alias|and|BEGIN|begin|break|case|class|def|define_method|defined|do|each|else|elsif|END|end|ensure|false|for|if|in|module|new|next|nil|not|or|protected|private|public|raise|redo|require|rescue|retry|return|self|super|then|throw|true|undef|unless|until|when|while|yield)\b/,
+		'function': {
+			pattern: /(def )[\w.]+/,
+			lookbehind: true,
+			greedy: true
+		}
 	});
 
 	var interpolation = {
@@ -128,4 +133,4 @@
 			}
 		}
 	];
-}(Prism));
+}(Prism));