Test out Cargo Crusader on Travis

[Gankra]

@brson is a magical wizard: https://github.com/brson/cargo-crusader

BitVec is a great candidate, I think.

[brson]

I'm not sure how useful this will be for CI as is, because it's slow and because it generates the report out-of-band. You'll definitely want to export the generated html file somewhere permanent for later reading.

[Gankra]

I'm not necessarily interested in the report, so much as "don't break everyone" -- novel use of semver may enable having it on unconditionally (e.g. regressions are fine if the PR bumps the version).

[Gankra]

However integration with e.g. homu is another option, for sure.