As Debian has been in feature freeze since before Mosquitto 1.0 was released, it will be a long time until there is an updated version of Mosquitto in Debian. It is, however, fairly straightforward to do the packaging yourself. Here's how to do that from the command line.
Download and unpack the mosquitto source tarball:
wget http://mosquitto.org/files/source/mosquitto-1.1.2.tar.gz tar -zxf mosquitto-1.1.2.tar.gz
Rename the tarball to match Debian requirements:
mv mosquitto-1.1.2.tar.gz mosquitto_1.1.2.orig.tar.gz
The current mosquitto packaging files are available at https://packages.debian.org/source/stable/mosquitto - you want the .debian.tar.xz.
The next step is to build the package, but you may find that you need to install some packages first:
sudo apt-get install build-essential python quilt libwrap0-dev libssl-dev devscripts python-setuptools
To build the packages do
cd mosquitto-1.1.2/ debuild
You should now have a list of .deb files in the parent directory which you can install with:
sudo dpkg -i <deb file>
Please leave comments if you find this useful or have any problems.