* move to cargo clippy instead of using the clippy plugin
fixes#729
* non-exectable scripts must be run with `sh`
* don't build serde in the clippy travis job
* only run clippy tests if installing clippy succeeds
* why is travis so picky?
* no more serde_codegen
* serde_test_suite_deps has no features
* don't use empty loops, llvm optimizes them to undefined behaviour
* abort the clippy job when clippy lints are triggered
* use caches on travis to speed up builds
* why are we even using `travis-cargo`?
* need to reinstall clippy frequently due to nightly updates
* command line tools are hard
David Tolnay