No output coverage file

Call istanbul.hookRequire() option.

gulp-istanbul document says:
`Always use this option if you're running tests in Node.js`
See:
https://github.com/SBoudrias/gulp-istanbul/blob/8c36677334c0c4957d019e52254d66d68ad05083/README.md
Bump gulp-istanbul to ^0.5.0
Merge pull request #50 from sanemat/fix/gulp-istanbul-setting

No output coverage file