Remove strict need for an issue in the PR template

There are some exceptions where an issue is not necessary

Closes gh-559.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 **Make sure these boxes are checked before submitting your Pull Request, thank you!**
 
 - [ ] I have read and complied with the [Pull Request Contributing Guidelines](https://github.com/impress/impress.js/blob/master/CONTRIBUTING.md#pull-requests).
-- [ ] I confirm there is [an issue](https://github.com/impress/impress.js/issues) for my Pull Request.
+- [ ] I checked if an [an issue](https://github.com/impress/impress.js/issues) is necessary for my Pull Request and made sure it exists.
 - [ ] I have created a [topic branch](https://github.com/dchelimsky/rspec/wiki/Topic-Branches).
 - [ ] I have used the [Search Tool](https://github.com/impress/impress.js/search?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen&type=Issues) to make sure there are no open Pull Requests fixing the same thing.