README.md: Add Travis CI badge
README.md: Add Travis CI badge

See here for details:
https://docs.travis-ci.com/user/status-images/

mathiasbynens/xregexp,GerHobbelt/xregexp,mathiasbynens/xregexp,slevithan/xregexp,mathiasbynens/xregexp,slevithan/xregexp,slevithan/XRegExp,slevithan/xregexp,GerHobbelt/xregexp,mathiasbynens/xregexp,slevithan/XRegExp,GerHobbelt/xregexp