Update to 2.66

Upstream changes:
2.66 2019-02-11
    - Full release of 2.65_001, with one additional definedness check (pajlpajl++)

2.65_001 2019-02-11
    - fix file_version method (it was calling the wrong key) (GH #23)
    - avoid repeated loading of files when getting headers
      and incorrect undef returns (GH #22)
    - both reported by pajlpajl

(wen)