chore(readme): replace buildhive with travis badge

README.md

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 camunda BPM - The open source BPM platform
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 camunda BPM platform is a flexible framework for workflow and process automation. It's core is a native BPMN 2.0 process engine that runs inside the Java Virtual Machine. It can be embedded inside any Java application and any Runtime Container. It integrates with Java EE 6 and is a perfect match for the Spring Framework. On top of the process engine, you can choose from a stack of tools for human workflow management, operations & monitoring.