move to cargo clippy instead of using the clippy plugin (#733)

* 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

serde/Cargo.toml

@@ -22,10 +22,7 @@ std = []
 unstable = []
 alloc = ["unstable"]
 collections = ["alloc"]
-unstable-testing = ["clippy", "unstable", "std"]
-
-[dependencies]
-clippy = { version = "0.*", optional = true }
+unstable-testing = ["unstable", "std"]
 
 [dev-dependencies]
 serde_derive = "0.9"