Upgrade dependencies (#14)

* Upgrading peer deps check, and fixing link in issues template

* Bumping all packages, so we are testing with webpack@4 and html-webpack-plugin@3, and fixing tests

Author: AnujRNair

.github/ISSUE_TEMPLATE.md

@@ -11,7 +11,7 @@ Describe your issue here.
 - [ ] discussion
 
 ### Requirements (place an `x` in each of the `[ ]`)
-* [ ] I've read and understood the [Contributing guidelines](https://github.com/slackhq/csp-html-webpack-plugin/blob/master/.github/contributing.md) and have done my best effort to follow them.
+* [ ] I've read and understood the [Contributing guidelines](https://github.com/slackhq/csp-html-webpack-plugin/blob/master/.github/CONTRIBUTING.md) and have done my best effort to follow them.
 * [ ] I've read and agree to the [Code of Conduct](https://slackhq.github.io/code-of-conduct).
 * [ ] I've searched for any related issues and avoided creating a duplicate issue.