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_codegen_internals/Cargo.toml

@@ -10,11 +10,7 @@ documentation = "https://docs.serde.rs/serde_codegen_internals/"
 keywords = ["serde", "serialization"]
 include = ["Cargo.toml", "src/**/*.rs"]
 
-[features]
-unstable-testing = ["clippy"]
-
 [dependencies]
-clippy = { version = "0.*", optional = true }
 syn = "0.11"
 
 [badges]