Invalid license specified

[harrystevenson]

The licence for this project isn't a proper SPDX expression.
It is currently BSD 3-clause but should be BSD-3-Clause

The complete list of SPDX licence expressions is found at https://spdx.org/licenses/

I'm happy to submit a PR to fix this, but it should just be a change to the licence field in the package.json.

[Tasyp]

It was fixed in #42. Thanks!