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

test_suite/no_std/src/main.rs

@@ -21,7 +21,9 @@ pub extern fn rust_eh_unwind_resume() {}
 pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
                                _file: &'static str,
                                _line: u32) -> ! {
-    loop {}
+    unsafe {
+        libc::abort()
+    }
 }
 
 //////////////////////////////////////////////////////////////////////////////