[ci] Updates to newer version of machine and python

.travis.yml

@@ -1,8 +1,8 @@
-dist: trusty
+dist: bionic
 sudo: required
 language: python
 python:
-- '3.6'
+- '3.7'
 cache:
   - pip
   - apt
@@ -23,6 +23,7 @@ addons:
     - lmodern
     - fonts-cmu
     - inkscape
+    - ffmpeg
 
 deploy:
   provider: pages
@@ -58,7 +59,4 @@ before_install:
 - eval `ssh-agent -s`
 - chmod 600 deploy_key
 - ssh-add deploy_key
-- sudo add-apt-repository -y ppa:mc3man/trusty-media
-- sudo apt-get -y update
-- sudo apt-get -y install ffmpeg
 after_success:

latex.tplx

@@ -56,5 +56,5 @@
 ((* endblock docclass *))
 
 ((* block markdowncell scoped *))
-((( cell.source | citation2latex | strip_files_prefix | markdown2latex(extra_args=["--chapters"]) )))
+((( cell.source | citation2latex | strip_files_prefix | markdown2latex(extra_args=["--top-level-division=chapter"]) )))
 ((* endblock markdowncell *))

nbmerge.py

@@ -13,7 +13,8 @@
 import string
 import sys
 
-from IPython import nbformat
+import nbformat
+
 def fix_images_paths(cells, filename):
     # find parent path
     path_filename = filename.split('/')