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/src/lib.rs

@@ -1,6 +1,3 @@
-#![cfg_attr(feature = "clippy", plugin(clippy))]
-#![cfg_attr(feature = "clippy", feature(plugin))]
-
 extern crate syn;
 
 pub mod ast;