update gulp version to 4.0

[immartian]

some issues including #1411 are all related to unsupported gulp v3.x, so changed the least version to v4.0.0 to assure the binding of several packages including Prism and Node-sass.

tested on both node v11.x and v12.x.

[immartian]

Without this change, the current code base will not be able to run, most probably showing such errors:

Aborted
npm ERR! code ELIFECYCLE
npm ERR! errno 134
npm ERR! stackedit@5.13.3 postinstall: `gulp build-prism`
npm ERR! Exit status 134
npm ERR! 
npm ERR! Failed at the stackedit@5.13.3 postinstall script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

[ProbablyFaiz]

+1, this fixed my local instance which was previously showing that 134 code you posted above.