CI: Download wx wheels for the correct Ubuntu version.

QuLogic · QuLogic · commit 73ed9a6aa172 · 2020-04-24T20:48:18.000-04:00
We are now on xenial, so should use the 16.04 URL.
diff --git a/.travis.yml b/.travis.yml
@@ -149,7 +149,7 @@ install:
       echo 'PySide2 is available' ||
       echo 'PySide2 is not available'
     python -mpip install --upgrade \
-      -f https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-14.04 \
+      -f https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-16.04 \
       wxPython &&
       python -c 'import wx' &&
       echo 'wxPython is available' ||
