Add new python versions to CI tests

.travis.yml

@@ -1,14 +1,16 @@
 # os: linux and sudo: false is assumed, which means it is using container-based Ubuntu
 language: python
 cache: pip
+dist: xenial   # required for Python >= 3.7
 # build matrix: different python and pandoc versions
 python:
   - "2.7"
   - "3.4"
   - "3.5"
   - "3.6"
-  - "3.6-dev" # 3.6 development branch
-  - "3.7-dev" # 3.7 development branch
+  - "3.7"
+  - "3.7-dev"  # 3.7 development branch
+  - "3.8-dev"  # 3.8 development branch
   - "nightly" # Travis CI supports a special version name nightly, which points to a recent development version of CPython build
   # pypy (version info from [Changelogs — PyPy documentation](http://doc.pypy.org/en/latest/index-of-whatsnew.html))
   - "pypy"