Rename LICENSE in order to be referred to README

A link to Apache 2.0 license `[![][license img]][license]` in README.md links to
a dead link https://github.com/Netflix/Hystrix/blob/master/LICENSE-2.0.txt actually, which might not be intended.

Author: nyango

CONTRIBUTING.md

@@ -6,7 +6,7 @@ When submitting code, please make every effort to follow existing conventions an
 
 ## License
 
-By contributing your code, you agree to license your contribution under the terms of the APLv2: https://github.com/Netflix/Hystrix/blob/master/LICENSE
+By contributing your code, you agree to license your contribution under the terms of the APLv2: https://github.com/Netflix/Hystrix/blob/master/LICENSE-2.0.txt
 
 All files are released with the Apache 2.0 license.