Fix license recognition on GitHub.com

[pchaigno]

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.

I've checked locally that, with this pull request, the Apache v2.0 license is correctly recognized:

$ licensee
License:        Apache-2.0
Matched files:  LICENSE.txt
LICENSE.txt:
  Content hash:  e801a9b4965d582939632e9c5f747024a68275ab
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       Apache-2.0 

Thus, with this pull request, bcc's license should be displayed at the top of the repository.