Remove travis addons that aren't needed and causing issues (#158)

Author: brianegan

.travis.yml

@@ -1,13 +1,6 @@
 os:
   - linux
 sudo: false
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - libstdc++6
-      - fonts-droid
 before_script:
   - git clone https://github.com/flutter/flutter.git -b stable --depth 1
   - ./flutter/bin/flutter doctor