If a plugin uses the core function "download_file_content", and is installed with a the 3.2 (Build 20161115) or later, it can fail. This is because that function now directly accesses $CFG->$curlsecurityallowedport. That variable is new with 3.2 (Build 20161115), and doesn't exist until the settings page for "HTTP Security" is saved. For a command line install like Travis, this settings page may not have been saved yet, and thus the install fails with an "undefined property" error.
I believe this could only happen with command line or automated installers, as it is difficult to avoid the settings save screen with an interactive install.
To prevent this, the variables existence should be checked before it as accessed.

This is a bug caused by MDL-48498