Trisquel 7 is based on Ubuntu 14.04. To upgrade HPLIP to version 3.15.11, you
could try to install by hand the package in the repository of Ubuntu 16.04
Xenial Xerus: http://packages.ubuntu.com/xenial/hplip
With GDebi installed (GDebi is in Trisquel's repository), you can just
download the package for your architecture (see the bottom of the Web page I
linked above) and double-click on the .deb. You will enter "dependency hell":
GDebi will notify dependencies that are missing (or whose version in
Trisquel's repository is too dated). You can then search and download those
packages from Xenial's repository. When trying to install those dependencies,
GDebi may signal other missing dependencies. And so on. Do not go too far
though: updating libc6 or even python3 will raise serious troubles... and
Ubuntu 16.04 is not even stable yet!
Isn't it enough to upgrade to HPLIP 3.15.2? It is in the repository of Ubuntu
15.04 Vivid Vervet: http://packages.ubuntu.com/vivid-updates/hplip
You would enter "dependency hell" as well but it should be easier (less
packages to update).
