The problem most probably appeared when Travis recently dropped container-based environments.
Unless your project language: is python, you only have whatever Python 2 and Python 3 versions the distribution provides preinstalled – which for Trusty is python3 3.4.0. If you need anything else, you have to install it yourself.
The simplest solution for you will be to move to dist: xenial which provides python3 3.5.1 that meson still supports as of this writing.
Note that you’ll have to sudo apt-get python3-setuptools python3-pip to install packages with pip3.