We’ve switched the default OS for new Linux projects to Ubuntu 14.04 Trusty.
This means that next time you add a new Linux project it will build on 14.04 by default. Please note that the change applies only to new Linux projects, so the OS of your existing projects will not be changed unless you choose to do so. You can also choose to run your new projects on Ubuntu 12.04 (Precise) under the Build Environment tab on your project’s settings page.
More details on the differences between 12.04 and 14.04 in our blog post announcing the change