Rename `MITL` to `LICENSE` to conform GitHub convention.

LEGAL

@@ -2,5 +2,5 @@ LEGAL NOTICE INFORMATION
 ------------------------
 
 All the files in this distribution are covered under the MIT license
-(see the file MITL) except some files mentioned below:
+(see the file LICENSE) except some files mentioned below:
 

MITL → LICENSE

-Copyright (c) 2018 mruby developers
+Copyright (c) 2019 mruby developers
 
 Permission is hereby granted, free of charge, to any person obtaining a
 copy of this software and associated documentation files (the "Software"),

README.md

@@ -59,7 +59,7 @@ how to use mrbgems look into the folder *examples/mrbgems/*.
 
 ## License
 
-mruby is released under the [MIT License](MITL).
+mruby is released under the [MIT License](LICENSE).
 
 ## Note for License