fix(CI): fix cargo-travis(kcov) error

Error description(see https://travis-ci.org/rust-bio/rust-htslib/builds/424648253):

```
kcov: warning: kcov: WARNING: kcov has been built without libbfd-dev (or
kcov: binutils-dev), so the --verify option will not do anything.
Can't set personality: Operation not permitted
kcov: error: Can't start/attach to /home/travis/build/rust-bio/rust-htslib/target/debug/deps/rust_htslib-b4e39d181972c8d6
Child hasn't stopped: ff00
kcov: error: Can't start/attach to /home/travis/build/rust-bio/rust-htslib/target/debug/deps/rust_htslib-b4e39d181972c8d6
error: test failed
```

Reference: roblabla/cargo-travis#38

.travis.yml

@@ -15,6 +15,7 @@ addons:
       - libelf-dev
       - libdw-dev
       - binutils-dev
+      - libiberty-dev
       - cmake
       - musl
       - musl-dev