Fix bug not respecting ruby version you pass to configure.

Use gdiff since our diff doesn't implement --strip-trailing-cr.

--- test/runtests.in.orig	Mon Oct 13 20:00:59 2014
+++ test/runtests.in	Sat May  9 16:56:59 2015
@@ -119,11 +119,11 @@ function run_test()
 		
 		exec_cmd=./$binary
 		[ $lang = java ] && exec_cmd="java ${root}"
-		[ $lang = ruby ] && exec_cmd="ruby ${code_src}"
+		[ $lang = ruby ] && exec_cmd="${ruby_engine} ${code_src}"
 		[ $lang = csharp ] && [ "$csharp_compiler" = gmcs ] && exec_cmd="mono ${exec_cmd}"
 
 		$exec_cmd 2>&1 > $output;
-		if diff --strip-trailing-cr $expected_out $output > /dev/null; then
+		if gdiff --strip-trailing-cr $expected_out $output > /dev/null; then
 			echo "passed";
 		else
 			echo "FAILED";