Change LICENSE to BSD-2-Clause.

As original implementation by @antirez now has this license.

See:
antirez/redlock-rb@97be08b
antirez/redlock-rb#3

LICENSE

@@ -0,0 +1,26 @@
+Copyright (c) 2014-2015, Salvatore Sanfilippo <antirez at gmail dot com>
+Copyright (c) 2014-2015, Leandro Moreira <leandro dot ribeiro dot moreira at gmail dot com>
+Copyright (c) 2015,      Malte Rohde <malte dot rohde at flavoursys dot com>
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

redlock.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{Distributed lock using Redis written in Ruby.}
   spec.description   = %q{Distributed lock using Redis written in Ruby. Highly inspired by https://github.com/antirez/redlock-rb.}
   spec.homepage      = "https://github.com/leandromoreira/redlock-rb"
-  spec.license       = "MIT"
+  spec.license       = 'BSD-2-Clause'
 
   spec.files         = `git ls-files -z`.split("\x0")
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }