Fix Ruby syntax highlighting for predefined variables (zyedidia#3778)

Author: Andriamanitra

runtime/syntax/ruby.yaml

@@ -21,6 +21,10 @@ rules:
     - constant.number: "(?i)\\b0x[0-9a-fA-F][0-9a-f_]*\\b"
     - constant.number: "(?i)\\b0b[01][01_]*\\b"
     - constant.number: "(?i)\\b[0-9][0-9_]*(['.'][0-9_]+)?(e[\\-]?[0-9_]+)?\\b"
+    # Predefined global variables
+    - constant:
+        start: "[$]([!@&`'+~=/\\\\,;.<>*$?:\"_]|-[A-Za-z0-9_]|[0-9]+)"
+        end: "\\B|\\b"
     # Ruby "Symbols"
     - constant: "(i?)([ 	]|^):[0-9A-Z_]+\\b"
     - constant: "\\b(__FILE__|__LINE__)\\b"