docs: updated debian distro in install docs to stretch, closes #11527

docs/sources/installation/debian.md

@@ -34,7 +34,7 @@ sudo dpkg -i grafana_5.0.4_amd64.deb
 Add the following line to your `/etc/apt/sources.list` file.
 
 ```bash
-deb https://packagecloud.io/grafana/stable/debian/ jessie main
+deb https://packagecloud.io/grafana/stable/debian/ stretch main
 ```
 
 Use the above line even if you are on Ubuntu or another Debian version.
@@ -42,7 +42,7 @@ There is also a testing repository if you want beta or release
 candidates.
 
 ```bash
-deb https://packagecloud.io/grafana/testing/debian/ jessie main
+deb https://packagecloud.io/grafana/testing/debian/ stretch main
 ```
 
 Then add the [Package Cloud](https://packagecloud.io/grafana) key. This