Add TensorFlow version to bug report template (#2771)

Author: Chris Elion

.github/ISSUE_TEMPLATE/bug_report.md

@@ -26,6 +26,7 @@ If applicable, add screenshots to help explain your problem.
 **Environment (please complete the following information):**
 - OS + version: [e.g. Windows 10]
 - _ML-Agents version_: (e.g. ML-Agents v0.8, or latest `develop` branch from source)
+- _TensorFlow version_: (you can run `pip3 show tensorflow` to get this)
 - _Environment_: (which example environment you used to reproduce the error)
 
 **NOTE:** We are unable to help reproduce bugs with custom environments.  Please attempt to reproduce your issue with one of the example environments, or provide a minimal patch to one of the environments needed to reproduce the issue.