Update to 3.5.1

Upstream changes:
v3.5.1 2017-11-30T16:09:33Z
    - Fix packaging issue; explicitly declare version for Text::Xslate::Engine

v3.5.0 2017-11-30T15:57:50Z
    - Migrate to using minil (#179)
    - Improve perl 5.8 support (#182)
    - Use version->declare() to declare $VERSION (#182)
    - Make `prove -br t` work with dot-not-in-INC perls (#184)
    - Run tests against both XS and pureperl (#185)
    - Fix typo (anirvan #181)

(wen)