Remove second polyfill for Array.forEach

[bitcloud]

There are two polyfills for Array.forEach.

The second one seems to be obsolete so I removed it.

iframe-resizer/src/ie8.polyfils.js

Line 48 in 2b2df4b

if (!Array.prototype.forEach) {

And I made the project a bit more self contained by installing grunt-cli as devDependency. No need for a global grunt-cli anymore ;-)
(sorry for putting them in one PR)

[coveralls]

Coverage remained the same at 94.106% when pulling a743c9f on bitcloud:polyfill into 2b2df4b on davidjbradshaw:master.

[davidjbradshaw]

Thanks for this

[davidjbradshaw]

Release as v3.5.16