Export flutter path

.travis.yml

 before_script:
   - git clone https://github.com/flutter/flutter.git -b $FLUTTER_VERSION
   - ./flutter/bin/flutter doctor
+  - export PATH="$PATH:./flutter/bin"
 
 script:
 - ./tool/travis.sh notus

packages/notus/lib/src/heuristics/format_rules.dart

     return result;
   }
 }
-

packages/zefyr/tool/travis.sh

 
 set -e
 
-~/flutter/bin/flutter test
+flutter test