Try meson/ninja from pypi

This lets us simplify linux a little.

However, we still want to test cmake, so there is only so much we
can simplify.

For OSX, we still need `clang-format` from homebrew.

* Add PYTHONUSERBASE/bin to PATH for linux

.travis_scripts/travis.before_install.osx.sh

@@ -1 +0,0 @@
-# NOTHING TO DO HERE

.travis_scripts/travis.install.linux.sh

@@ -1,10 +1,5 @@
 set -vex
 
-wget https://github.com/ninja-build/ninja/releases/download/v1.9.0/ninja-linux.zip
-unzip -q ninja-linux.zip -d build
-
-pip3 install meson
-echo ${PATH}
-ls /usr/local
-ls /usr/local/bin
-export PATH="${PWD}"/build:/usr/local/bin:/usr/bin:${PATH}
+pip3 install --user meson ninja
+which meson
+which ninja