URLs look like this:
http://arduino.esp8266.com/versions/1.6.5-1106-g8253b82/package_esp8266com_index.json
Substitute the version number with the one you want.
Here's my release log for your reference:
Thu Jul 23 17:54:43 MSK 2015: changing version of staging from versions/1.6.5-942-gc6e2a29 to versions/1.6.5-946-g6b3ed67
Thu Jul 23 19:03:15 MSK 2015: changing version of stable from versions/1.6.4-673-g8cd3697 to versions/1.6.5-947-g39819f0
Tue Jul 28 20:17:02 MSK 2015: changing version of staging from versions/1.6.5-946-g6b3ed67 to versions/1.6.5-981-gbfc6b27
Thu Jul 30 13:53:07 MSK 2015: changing version of staging from versions/1.6.5-981-gbfc6b27 to versions/1.6.5-987-g71b2166
Thu Jul 30 13:59:15 MSK 2015: changing version of staging from versions/1.6.5-987-g71b2166 to versions/1.6.5-988-gfd1d9da
Mon Aug 3 14:09:13 EDT 2015: changing version of staging from versions/1.6.5-988-gfd1d9da to versions/1.6.5-990-gc8a63ce
Mon Aug 10 19:03:38 MSK 2015: changing version of staging from versions/1.6.5-990-gc8a63ce to versions/1.6.5-1044-g170995a
Mon Aug 31 10:54:35 MSK 2015: changing version of staging from versions/1.6.5-1044-g170995a to versions/1.6.5-1084-ga39ce29
Wed Sep 9 18:11:43 MSK 2015: changing version of staging from versions/1.6.5-1084-ga39ce29 to versions/1.6.5-1105-g98d2458
Wed Sep 9 18:18:57 MSK 2015: changing version of staging from versions/1.6.5-1105-g98d2458 to versions/1.6.5-1106-g8253b82