Install ant-optional in Travis CI builds (beta)
Install ant-optional in Travis CI builds (beta)

Change .travis.yml to install the package ant-optional, required to run
the tests with newer dist trusty.

secdec/zap-extensions,zaproxy/zap-extensions,veggiespam/zap-extensions,denniskniep/zap-extensions,veggiespam/zap-extensions,kingthorin/zap-extensions,secdec/zap-extensions,secdec/zap-extensions,veggiespam/zap-extensions,thc202/zap-extensions,kingthorin/zap-extensions,kingthorin/zap-extensions,psiinon/zap-extensions,psiinon/zap-extensions,denniskniep/zap-extensions,psiinon/zap-extensions,thc202/zap-extensions,psiinon/zap-extensions,zaproxy/zap-extensions,psiinon/zap-extensions,zapbot/zap-extensions,zapbot/zap-extensions,zapbot/zap-extensions,thc202/zap-extensions,denniskniep/zap-extensions,denniskniep/zap-extensions,zaproxy/zap-extensions,zaproxy/zap-extensions,secdec/zap-extensions,veggiespam/zap-extensions,kingthorin/zap-extensions,secdec/zap-extensions,veggiespam/zap-extensions,secdec/zap-extensions,zaproxy/zap-extensions,thc202/zap-extensions,denniskniep/zap-extensions,kingthorin/zap-extensions,zaproxy/zap-extensions,secdec/zap-extensions,zapbot/zap-extensions,kingthorin/zap-extensions,psiinon/zap-extensions,denniskniep/zap-extensions,zapbot/zap-extensions,thc202/zap-extensions,zapbot/zap-extensions,veggiespam/zap-extensions,thc202/zap-extensions