Modernize .travis.yml, add Python 3.6/Bionic (#3218)

Upgrade the Travis YAML control file to:
- Use the 'language: python' test environment
- Install deps with the Travis apt plugin, instead of sudo
- Install cx_Freeze with pip3, rather than from a URL
- Add a Python 3.6 test build, running on Bionic

.travis.yml

-language: cpp
-dist: xenial
-sudo: required
+# Run in default virtualenv, but with access to Python packages
+# installed by apt
+language: python
+virtualenv:
+  system_site_packages: true
+addons:
+  apt:
+    sources:
+    - sourceline: 'ppa:openshot.developers/libopenshot-daily'
+    packages:
+    - libopenshot-audio-dev
+    - libopenshot-dev
+    - libssl-dev
+    - python3-openshot
+    - python3-pip
+    - python3-setuptools
+    - python3-pyqt5
+    - python3-pyqt5.qtmultimedia
+    - python3-pyqt5.qtopengl
+    - python3-pyqt5.qtsvg
+    - python3-pyqt5.qtwebkit
+    - python3-requests
+    - python3-xdg
+    - python3-zmq
+    - qttranslations5-l10n
+    - xvfb
 
-before_install:
-  - sudo add-apt-repository ppa:openshot.developers/libopenshot-daily -y
-  - sudo apt-get update -qq
-  - sudo apt-get install gcc-4.8 xvfb tar cmake libopenshot-dev libopenshot-audio-dev libunittest++-dev swig doxygen doxypy libssl-dev python3 python3-dev python3-pip python3-pyqt5 python3-setuptools python3-openshot python3-pyqt5 python3-pyqt5.qtmultimedia python3-pyqt5.qtopengl python3-pyqt5.qtsvg python3-pyqt5.qtwebkit python3-requests python3-xdg python3-zmq qttranslations5-l10n -y
-  - sudo apt autoremove -y
-  - wget https://files.pythonhosted.org/packages/d0/6d/9492644452727094543575de9846af5a2b9c764f760e2d414af7c876618c/cx_Freeze-5.0.1.tar.gz
-  - tar xf cx_Freeze-5.0.1.tar.gz
-  - cd cx_Freeze-5.0.1; python3 setup.py build; sudo python3 setup.py install; cd ..;
-  - pwd
+matrix:
+  include:
+    - name: "Python 3.5 (Ubuntu 16.04 Xenial)"
+      os: linux
+      dist: xenial
+      python: 3.5
+    - name: "Python 3.6 (Ubuntu 18.04 Bionic)"
+      os: linux
+      dist: bionic
+      python: 3.6
+
+install:
+  - pip3 install cx-freeze
 
 script:
-  - python3 freeze.py build;
+  - python3 freeze.py build
   - xvfb-run --auto-servernum --server-num=1 --server-args "-screen 0 1920x1080x24" python3 src/tests/query_tests.py
 
 notifications:
@@ -22,4 +49,4 @@ notifications:
 env:
   global:
   - TRAVIS_LIBOPENSHOT=TEST
-  - LANG="en_US.UTF-8"
\ No newline at end of file
+  - LANG="en_US.UTF-8"