fix(benches): adjust to missing set_ssl_verifier

In 53bba6e , ssl usage was improved. The existing benchmark
implementation wasn't adjusted though.

This commit fixes the client benchmark to work with the latest nightly
rust and instructs travis to try compile the benchmarks
when rustc nightly is used. This should prevent such kind of breakage
in future.

.travis.yml

@@ -20,6 +20,7 @@ cache:
 script:
   - cargo build --verbose $FEATURES
   - cargo test --verbose $FEATURES
+  - '[ $TRAVIS_RUST_VERSION = nightly ] && cargo bench --no-run || :'
 
 addons:
   apt: