travis-ci: pin cbindgen to 0.14.1

0.14.2 breaks builds with Rust 1.34.0, which we still support.

Also build cbdingen in debug mode. It builds much faster
with minimal runtime performance.

.travis.yml

@@ -199,5 +199,5 @@ before_install:
     rustup default "${RUST_VERSION}"
     rustc --version
 
-    cargo install --force cbindgen
+    cargo install --force --debug --version 0.14.1 cbindgen
   - ./qa/travis-libhtp.sh