travis: install wrk

.travis.yml

@@ -2,7 +2,10 @@ language: node_js
 node_js:
   - "4"
   - "5"
-sudo: false
+before_script:
+  - sudo apt-get install build-essential libssl-dev git
+  - git clone https://github.com/wg/wrk.git
+  - cd wrk && make && sudo cp wrk /usr/local/bin && cd ..
 script:
   - make test-travis
   - make bench