Merge branch 'hotfix/3345'

Close #3345
Added --prefer-source flag to composer install command

- GitHub now wants zipball downloads to go through the API, which quickly runs
  into limits. Adding the --prefer-source flag circumvents that at the expense
  of a costlier clone operation.
Merge branch 'hotfix/travis-composer-source' into develop

Make builds more reliable until travis-ci can get increased API limits for
download zipballs