source_location should be set config enable_debug

.travis_build_config.rb

@@ -1,5 +1,6 @@
 MRuby::Build.new do |conf|
   toolchain :gcc
+  enable_debug
   conf.gembox 'default'
   conf.gem '../mruby-method'
 end

README.md

@@ -11,3 +11,13 @@ p Enumerable.instance_method(:find_all).source_location
 ```
 
 ![https://raw.githubusercontent.com/ksss/mruby-method/master/find_all.png](https://raw.githubusercontent.com/ksss/mruby-method/master/find_all.png)
+
+# Note
+
+`source_location` method need this configuration in build_config.rb
+
+```
+MRuby::Build.new do |conf|
+  enable_debug
+end
+```