Issue title: !+'\v1'>>>!1
 Issue body: !+'\v1' >>> !1 

var ua = navigator.userAgent;
var c = function (r){
    return r.test(ua);
},
isIE = !+'\v1',
isTrident = isIE || !!global.XDomainRequest,
isGecko = !!document.getBoxObjectFor || global.mozInnerScreenX != null,
isWebkit = !isTrident &&  !navigator.taintEnabled && !c(/ Firefox/);

 Comments: 
 Comment 0: Is there a bug here? 

 Comment 1: Yes, " !+'\v1' "  to  " !1 "  After the uglify

 Comment 2: var ua = navigator.userAgent;
var c = function (r) {
        return r.test(ua);
    },
    isIE = !+'\v1',
    isTrident = isIE || !!global.XDomainRequest,
    isGecko = !!document.getBoxObjectFor || global.mozInnerScreenX != null,
    isWebkit = !isTrident && !navigator.taintEnabled && !c(/ Firefox/);

After the uglify:

var ua=navigator.userAgent,c=function(e){return e.test(ua)},isIE=!1,isTrident=isIE||!!global.XDomainRequest,isGecko=!!document.getBoxObjectFor||null!=global.mozInnerScreenX,isWebkit=!isTrident&&!navigator.taintEnabled&&!c(/ Firefox/);

 Comment 3: This is an "issue" with UglifyJS2, not this project, and it something unlikely to be resolved by them as this is the correct behavior.

Look into a new IE check.

{'source': 'GithubIssues', 'repo_name_with_owner': 'terinjokes/gulp-uglify', 'issue_id': 37781117, 'repo_stars': 1234.0}