Travis: run mypy with Python 3.8 as well (#3146)

This runs mypy both with Python 3.7 and 3.8. In Python 3.8,
mypy switched from using typed-ast to using Python's built-in ast.
This patch ensures that both are tested.

.travis.yml

@@ -1,4 +1,4 @@
-dist: xenial
+dist: bionic
 language: python
 python: 3.7
 
@@ -9,7 +9,12 @@ matrix:
       env:
         - TEST_CMD="./tests/pytype_test.py"
         - INSTALL="test"
-    - name: "mypy"
+    - name: "mypy (typed-ast)"
+      env:
+        - TEST_CMD="./tests/mypy_test.py"
+        - INSTALL="mypy"
+    - name: "mypy (ast)"
+      python: 3.8-dev
       env:
         - TEST_CMD="./tests/mypy_test.py"
         - INSTALL="mypy"