Temporarily disable coverage as it is broken in Flutter; added more envs to build

.travis.yml

 language: dart
 
 dart:
+  - stable
   - dev
 
 os:
     - $HOME/.pub-cache
 
 env:
-#  - FLUTTER_VERSION=beta
+  - FLUTTER_VERSION=beta
   - FLUTTER_VERSION=dev
 
 matrix:
 - pwd
 - ./tool/travis.sh notus
 - ./tool/travis.sh zefyr
-- bash <(curl -s https://codecov.io/bash)
+#- bash <(curl -s https://codecov.io/bash)

packages/zefyr/tool/travis.sh

 
 set -e
 
-$TRAVIS_BUILD_DIR/flutter/bin/flutter test --coverage
+$TRAVIS_BUILD_DIR/flutter/bin/flutter test