Fix license detection

[leoheitmannruiz]

Hey :)

Just a minor thing.

Currently licensee, the program used by GitHub to detect licenses, is not confident enough about what license is used:

License:        NOASSERTION
Matched files:  LICENSE
LICENSE:
  Content hash:  5227778a1739d9dabbcc2778eb28743ac1a96b8d
  License:       NOASSERTION
  Closest non-matching licenses:
    BSD-2-Clause similarity:  93.90%
    BSD-3-Clause similarity:  75.47%
    BSD-4-Clause similarity:  63.69%

This is because the disclaimer is slightly different from the one published by SPDX.

Now:

License:        BSD-2-Clause
Matched files:  LICENSE
LICENSE:
  Content hash:  a307ca750c21f262c95d2bce55e69c05e6c3dee9
  Attribution:   Copyright 2015-2019 Homebrew contributors.
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       BSD-2-Clause

[MikeMcQuaid]

Thanks again @leoheitmannruiz!