chore: update build URL and build status icon
elastic-apm -- Elastic APM agent for Python
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-.. image:: https://api.travis-ci.org/elastic/apm-agent-python.svg?branch=master
- :target: https://travis-ci.org/elastic/apm-agent-python
+.. image:: https://apm-ci.elastic.co/buildStatus/icon?job=apm-agent-python%2Fapm-agent-python-mbp%2Fmaster
+ :target: https://apm-ci.elastic.co/job/apm-agent-python/job/apm-agent-python-mbp/
:alt: Build Status
.. image:: https://img.shields.io/pypi/v/elastic-apm.svg?style=flat
