/status: fix blank distro version field.

The `source` command is a bash-ism and doesn't work in a strictly POSIX
shell like dash, which is the /bin/sh on Debian/Ubuntu. Use `.` instead.

https://en.wikipedia.org/wiki/Dot_(command)

app/logical/server_status.rb

@@ -46,7 +46,7 @@ class ServerStatus
     end
 
     def distro_version
-      `source /etc/os-release; echo "$NAME $VERSION"`.chomp
+      `. /etc/os-release; echo "$NAME $VERSION"`.chomp
     end
 
     def libvips_version