While investigating we looked at the latest commits, focusing on b0eec87bc3256bbc2112fd1611524817baf52364.
The enum change was ruled out as this is part of the server only (directory placement fixed in https://github.com/wmde/wikibase-termbox/pull/112) and not bundled into the client code.
Another change in the same patch seemed more likely to be the cause. We can not at the moment fully explain why, but removing the NODE_ENV=production env variable from the build-server script or deleting node_modules/.cache between the client and server build run makes for successful builds, i.e. the client bundle built on travis matches the one we built and checked-in locally.

https://github.com/wmde/wikibase-termbox/pull/114