Update Travis CI build status image branch name to php5.
Update Travis CI build status image branch name to php5.

metaclassing/phpseclib,melya/phpseclib,terrafrost/phpseclib,kenichiii/phpseclib,danog/phpseclib,metaclassing/phpseclib,danog/phpseclib,paragonie-scott/phpseclib,splitice/phpseclib,paragonie-scott/phpseclib,splitice/phpseclib,melya/phpseclib,creatuity/phpseclib,phpseclib/phpseclib,creatuity/phpseclib,thaigialai1987/phpseclib,thaigialai1987/phpseclib,kenichiii/phpseclib