ci/travis: skip pip3 upgrade if pip3 is missing

Workaround for travis issue:
https://github.com/travis-ci/travis-ci/issues/8363

Sometimes `pip3` works, sometimes not:
   pyenv: pip3: command not found
   The `pip3' command exists in these Python versions:
     3.5
     3.5.3

Tried these steps to fix the issue:
- add `python: 3.6` to top level of `.travis.yml`
- add `python3` to `addons.apt.packages` level of `.travis.yml`
- `pyenv global system 3.{4,5,6}`
- `pyenv global 3.6`
In all cases the presence or absence of `pip3` was random.

ci/before_install.sh

@@ -32,6 +32,10 @@ if [[ "${TRAVIS_OS_NAME}" == osx ]]; then
   echo "Upgrade Python 3 pip."
   pip3 -q install --user --upgrade pip
 else
-  echo "Upgrade Python 3 pip."
-  pip3 -q install --user --upgrade pip
+  if command -v pip3 ; then
+    echo "Upgrade Python 3 pip."
+    pip3 -q install --user --upgrade pip
+  else
+    echo 'warning: missing pip3'
+  fi
 fi