Replace Substrate mentions in license headers with Polkadot (#674)

executor/src/lib.rs

 // Copyright 2017 Parity Technologies (UK) Ltd.
-// This file is part of Substrate.
+// This file is part of Polkadot.
 
 // Substrate is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by

runtime/build.rs

 // Copyright 2019 Parity Technologies (UK) Ltd.
-// This file is part of Substrate.
+// This file is part of Polkadot.
 
 // Substrate is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by

runtime/src/claims.rs

 // Copyright 2017-2018 Parity Technologies (UK) Ltd.
-// This file is part of Substrate.
+// This file is part of Polkadot.
 
 // Substrate is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by

test-parachains/adder/build.rs

 // Copyright 2019 Parity Technologies (UK) Ltd.
-// This file is part of Substrate.
+// This file is part of Polkadot.
 
 // Substrate is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by

test-parachains/halt/build.rs

 // Copyright 2019 Parity Technologies (UK) Ltd.
-// This file is part of Substrate.
+// This file is part of Polkadot.
 
 // Substrate is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by