Use selective caching in travis builds

[juliangehring]

Caches the cargo binaries in travis builds which avoids recompiling cargo-travis and friends in every travis run. This saves ~10min in each run while keeping the cache small (<20MB).
See also:

• https://levans.fr/rust_travis_cache.html
• https://github.com/roblabla/cargo-travis

[coveralls]

Pull Request Test Coverage Report for Build 637

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.003%) to 60.062%

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Totals
Change from base Build 635:	-0.003%
Covered Lines:	21405
Relevant Lines:	35638

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💛 - Coveralls

[juliangehring]

The decrease in coverage seems to be an artefact and unrelated to the PR.

[johanneskoester]

Great, thank you!