travis CI: update meson image to one with a python that isn't EOL

travis CI: update meson image to one with a python that isn't EOL

Currently the build errors out with:

```
ERROR: This script does not work on Python 3.6 The minimum supported Python version is 3.7. Please use https://bootstrap.pypa.io/pip/3.6/get-pip.py instead.
```

While in theory this advice could be followed to get a better pip on
xenial, Meson has now deprecated python 3.6 support too, and the next
(unreleased) version requires python 3.7

There are a couple solutions to this:
- hold the version of pip, allow pip to only install 3.6-compatible
  versions of meson (effectively freezing meson going forward)
- install python 3.7 on xenial
- update to a 2-year-old image instead of a 4-year-old one

Option 3 is the simplest.