change SPDX identifier as the "license" field in "package.json"

[kret13]

Module has a wrong SPDX identifier as the "license" field in "package.json".
According to spdx list "BSD" is not a valid spdx identifier.

[jayvdb]

#23 has been merged, but not yet released to https://www.npmjs.com/package/css-mediaquery