Fix license recognition on GitHub.com

The second file with the license information, COPYRIGHT.txt, contains
the same information as LICENSE.txt, expect for the license's text.
However, it prevents Licensee, the tool used by GitHub to detect
licenses, from working properly as it doesn't know which file to take
into account.

COPYRIGHT.txt

-Copyright 2015 PLUMgrid
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.