Don't abort build when clippy fails

Before, if the `clippy` check would fail `exit 0` would be called which
exited the test script. This meant that all subsequent steps would be
skipped and the test would pass.

Now instead of exiting the test script we just make sure that the script
always return a zero exit code.

.travis.yml

@@ -13,7 +13,7 @@ before_script:
   - rustup target add thumbv7em-none-eabihf     # Any target that does not have a standard library will do
 script:
   - cargo fmt --all -- --check
-  - (rustup component add clippy && cargo clippy --all --all-features -- -D clippy::all) || exit 0
+  - (rustup component add clippy && cargo clippy --all --all-features -- -D clippy::all) || true
   - cargo build
   - cargo test
   - cargo build --no-default-features --target thumbv7em-none-eabihf # Test we can build a platform that does not have std.