licensing: add MPL-2.0+ as equivalent to MPL-2.0
    
    Permission to use later versions is included in the license, so the
    "-or-later" part does not change anything.
    https://fedoraproject.org/wiki/Licensing/MPLv2.0 says:
    > 10.2. Effect of New Versions
    >
    > You may distribute the Covered Software under the terms of the version
    > of the License under which You originally received the Covered
    > Software, or under the terms of any subsequent version published by
    > the license steward.
    
    Fixes #88.