Remove ant-optional workaround in Travis CI build
Remove ant-optional workaround in Travis CI build

Change .travis.yml to remove the manual installation of the package
ant-optional, it's now (update 2017-09-06) installed by default.

psiinon/zaproxy,Ali-Razmjoo/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,kingthorin/zaproxy,meitar/zaproxy,thc202/zaproxy,psiinon/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,gsavastano/zaproxy,zapbot/zaproxy,gmaran23/zaproxy,thc202/zaproxy,meitar/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,gsavastano/zaproxy,kingthorin/zaproxy,gmaran23/zaproxy,gsavastano/zaproxy,meitar/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,Ali-Razmjoo/zaproxy,thc202/zaproxy,zapbot/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,zaproxy/zaproxy,thc202/zaproxy,zaproxy/zaproxy,meitar/zaproxy,gsavastano/zaproxy,kingthorin/zaproxy,zaproxy/zaproxy,gmaran23/zaproxy,psiinon/zaproxy,psiinon/zaproxy,gsavastano/zaproxy,zapbot/zaproxy,meitar/zaproxy,psiinon/zaproxy,thc202/zaproxy,zapbot/zaproxy,meitar/zaproxy,psiinon/zaproxy,zapbot/zaproxy,meitar/zaproxy,Ali-Razmjoo/zaproxy,meitar/zaproxy,zaproxy/zaproxy,gsavastano/zaproxy,zapbot/zaproxy,thc202/zaproxy,zaproxy/zaproxy,gsavastano/zaproxy,Ali-Razmjoo/zaproxy,zapbot/zaproxy,gmaran23/zaproxy,gmaran23/zaproxy