Correct FFI version in legal.md and 3rd_party_licenses.txt

Author: eregon

3rd_party_licenses.txt

@@ -1308,7 +1308,7 @@ of
 */
 ================================================================================
 
-ffi 1.14.2
+ffi 1.15.5
 
 Copyright (c) 2008-2016, Ruby FFI project contributors
 All rights reserved.

doc/legal/legal.md

@@ -165,9 +165,8 @@ files.
 
 #### FFI
 
-TruffleRuby includes parts of the FFI gem 1.14.2. The FFI gem is copyright
-2008-2016, Ruby FFI project contributors, and covered by the three-clause BSD
-licence (see `ffi.txt`).
+TruffleRuby includes parts of the FFI gem (version as described in [lib/truffle/ffi/version.rb](../../lib/truffle/ffi/version.rb)).
+The FFI gem is copyright  2008-2016, Ruby FFI project contributors, and covered by the three-clause BSD licence (see `ffi.txt`).
 
 # Java dependencies