Thought I will elaborate a little bit on this one:
This repo https://github.com/Alfresco/activiti-spring-boot-samples is created as an example for Alfresco Enterprise customers to download the artifacts from Alfresco Nexus repository. If you are an Alfresco customer, please raise a case via your customer/partner portal to obtain the Nexus username/pw.
If you are not an Alfresco customer and just want to develop applications using the opensource libraries, as Ryan pointed out, please use the examples from https://github.com/Activiti/Activiti/tree/6.x/modules/activiti-spring-boot/spring-boot-samples
If you are wondering what the differences are between artifacts served from the open maven central vs private Alfresco Nexus repo - the artifacts from private repo contains some additional license check, patches/fixes made for customers, additional enterprise add-ons etc