Add @autoreleasepool keyword to Objective-C lexer (#1424)

This commit adds the `@autoreleasepool` keyword to the list recognised
by the Objective-C lexer.

lib/rouge/lexers/objective_c/common.rb

@@ -10,7 +10,7 @@ def at_keywords
         @at_keywords ||= %w(
           selector private protected public encode synchronized try
           throw catch finally end property synthesize dynamic optional
-          interface implementation import
+          interface implementation import autoreleasepool
         )
       end
 

spec/visual/samples/objective_c

@@ -29,6 +29,10 @@
 
 @end
 
+@autoreleasepool {
+    NSString *string = [[NSString alloc] initWithUTF8String:"I will be autoreleased"];
+}
+
 double (^multiplyTwoValues)(double, double) = ^(double firstValue, double secondValue) {
     return firstValue * secondValue;
 };