[AIRFLOW-39] Don't insert dag_runs beyond the min task end_date

[r39132]

Dear Airflow Maintainers,

Please accept this PR that addresses the following issues:

• AIRFLOW-39

Reminders for contributors:

• Your PR's title must reference an issue on Airflow's JIRA. For example, a PR called "[AIRFLOW-1] My Amazing PR" would close JIRA issue Improving the search functionality in the graph view #1. Please open a new issue if required!
• You must add an Apache License header to all new files.
• Please squash your commits when possible and follow the 7 rules of good Git commits.

[criccomini]

+1

[bolkedebruin]

+1