Make servlet-api a provided dependency

Tomcat is complaining about the included servlet-api-2.5.jar in the Hystrix Dashboard.

    INFO: validateJarFile(/opt/apache-tomcat-7.0.33/webapps/hystrix-dashboard-1.1.8-SNAPSHOT/WEB-INF/lib/servlet-api-2.5.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class

I've changed it to provided as done in https://github.com/Netflix/Hystrix/pull/50 for other subprojects.