Dart 2.8 version bump (#503)

diff --git a/.travis.yml b/.travis.yml
index f0a5ec6..33f2b5d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,7 +4,7 @@
 dart:
   # Keep this value aligned with min SDK value in pubspec.yaml and FROM line of Dockerfile
   - "2.7.2"
-  - "2.8.0-dev.19.0"
+  - "2.8.0-dev.20.0"
 
 # Speed up builds by using containerization. Disable this if you need to use
 # sudo in your scripts.