Links with titles not handled properly

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [x] Building sites locally

### What did you do

1. Setup jekyll and pages-gem
2. Write a markdown link with title to another local markdown file
- e.g. write `[support guideline](docs/SUPPORT.md "Read it first!")` into `README.md`
3. Generate htmls

### What did you expect to happen?

- The link goes from `.md` to `.html` in the generated html

### What happened instead?

- The link still points `.md` file

### Additional information

* It's caused by `jekyll-relative-links v0.6.1` plugin, and fixed by [`v0.7.0`](https://github.com/benbalter/jekyll-relative-links/releases/tag/v0.7.0)

Controlla la dichiarazione della dipendenza di pages-gem per jekyll-relative-links, concentrandoti sull'aggiornamento da v0.6.1 a v0.7.0 identificato nel report. Verifica il risultato generando un sito contenente un link con titolo da README.md a un file locale docs/SUPPORT.md e conferma che il link generato utilizzi il percorso .html.

Scritto dal modello di indicizzazione a partire dal testo della issue.