More robust check whether yacc should be build or not

This pull request does two things:

    - if there is a usuable binary (such as shipped within the git repo)
      it will be used. If for whatever reason that binary is not
      executable it will compile yacc from source
    - If the software is build within travis it will delete the binary
      and always use yacc from source

Note: _Please_ make any reasonable efforts to enable travis-ci on the RIPE-NCC/whois project. It makes life so much easier for anyone who would like to submit a pull request. It will also save the RIPE NCC considerable time: potential developers will be more self-reliant because the travis-ci workflow gives near instant feedback about the build quality of the proposed code changes.