Bump deps / Do not install gulp globally on CI

* I updated [gulp-istanbul](https://github.com/SBoudrias/gulp-istanbul).
* I replaced [del](https://github.com/sindresorhus/del) with [rimraf](https://github.com/isaacs/rimraf).
  * In that case, `del` is overkilling.
* I removed `./node_modules/.bin/` from the test command.
  * According to [the npm documentation](https://www.npmjs.org/doc/misc/npm-scripts.html#path):
    > If you depend on modules that define executable scripts, like test suites, then those executables will be added to the `PATH` for executing the scripts.