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/Cargo.toml

@@ -13,11 +13,8 @@ publish = false
 
 [features]
 unstable-testing = [
-    "clippy",
     "compiletest_rs",
     "serde/unstable-testing",
-    "serde_derive/unstable-testing",
-    "serde_test/unstable-testing",
 ]
 
 [dev-dependencies]
@@ -28,7 +25,6 @@ serde_derive = { path = "../serde_derive" }
 serde_test = { path = "../serde_test" }
 
 [dependencies]
-clippy = { version = "0.*", optional = true }
 compiletest_rs = { version = "0.2", optional = true }
 
 [[test]]