'dist:trusty' added to '.travis.yml' file

Author: mikechoch

.travis.yml

@@ -1,6 +1,7 @@
 os:
   - linux
 sudo: false
+dist: trusty
 addons:
   apt:
     # Flutter depends on /usr/lib/x86_64-linux-gnu/libstdc++.so.6 version GLIBCXX_3.4.18
@@ -16,4 +17,4 @@ script:
   - ./flutter/bin/flutter test
 cache:
   directories:
-    - $HOME/.pub-cache
+    - $HOME/.pub-cache