fix new import style in remap-istanbul

gulpfile.js

@@ -7,7 +7,7 @@ var merge = require("merge2")
 var sourcemaps = require("gulp-sourcemaps")
 var istanbul = require("gulp-istanbul")
 var mocha = require("gulp-mocha")
-var remapIstanbul = require("remap-istanbul/lib/gulpRemapIstanbul")
+var remapIstanbul = require("remap-istanbul/lib/gulpRemapIstanbul").default
 var coveralls = require("gulp-coveralls")
 var newer = require("gulp-newer")
 var replace = require("gulp-replace")