chore(travis): fix the build

I don’t know what the issue really is. There is some weird race condition, where the task copies the file (`pubspec.yml`) and then start `pub get` which reads that file and the file is not there or is empty. I have no idea why moving this task into a separate process fixes the issue but I don’t wanna waste more time on it.

Author: vojtajina

scripts/travis/build.sh

@@ -8,6 +8,10 @@ SCRIPT_DIR=$(dirname $0)
 cd $SCRIPT_DIR/../..
 source ./scripts/env.sh
 
+# For some reason, this task fails on Travis when run as a part of the `gulp build`.
+# It runs `pub get` which fails to read the `pubspec.yml` (created earlier by the task).
+./node_modules/.bin/gulp modules/build.dart/pubspec
+
 ./node_modules/.bin/gulp build
 
 pub install