Bump gtfs-lib version for flex

[x] Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
[x] Any modified or new methods or classes have helpful JavaDoc and code is thoroughly commented
[x] The description lists all applicable issues this PR seeks to resolve
[x] The description lists any configuration setting(s) that differ from the default settings
[x] All tests and CI builds passing

Bumping GTFS lib for flex to include recent stop headsign additions

ibi-group / datatools-server