As mentioned in the answer by BillGates, Aircrack-ng 1.1 is in the repos.

To install 1.2-beta3, you need to download the source:

wget http://download.aircrack-ng.org/aircrack-ng-1.2-beta3.tar.gz

Extract it:

tar xzf aircrack-ng-1.2-beta3.tar.gz

Change into the directory:

cd aircrack-ng-1.2-beta3

Make sure you have the build-essentials package installed to be able to compile programs. Then start compiling:

make

You might encounter some missing libraries along the way, for example you might see something like this:

common.mak:85: *** Cannot find development files for any supported version of libnl. install either libnl1 or libnl3..  Stop.

To solve these errors, install all missing development libraries, for example, libnl-3-dev for this one.

When it's done, you can run Aircrack from this directory or install it system-wide:

sudo make install