Fix: Create body for abstract methods (fixes eslint#80)

Author: soda0289

Diff for: lib/ast-converter.js

@@ -1140,6 +1140,9 @@ module.exports = function(ast, extra) {
                         });
                         if (isAbstractMethod) {
                             methodDefinitionType = "TSAbstractMethodDefinition";
+                            method.body = {
+                                type: "AbstractStatement"
+                            };
                         }
                     }