Update heroku to 3.9.1

[rpaulo]

Summary

Update heroku to 3.9.1

[swills]

Looks fine, but have you build tested this? If so, can you post a link to the logs? Also, a proposed commit message is nice to see in reviews.

[rpaulo]

In D398#3, @swills wrote:

Looks fine, but have you build tested this? If so, can you post a link to the logs? Also, a proposed commit message is nice to see in reviews.

Yes, it builds fine and it works. These are all bug fix releases.

3.9.1 2014-07-08

Support for HEROKU_DEBUG to show backtrace even on interrupt

3.9.0 2014-07-02

Updated bundled SSL certs
Added pg:diagnose

3.8.4 2014-06-18

Fix issue with rest-client dependency
Fix issue parsing errors from old API endpoints

3.8.3 2014-06-11

Replace Cisaurus slug copy with v3 Releases API

3.8.2 2014-05-27

Rename stack:migrate to stack:set
Bug fixes for the certs commands and another issue with 1.8 compatibility

3.8.1 2014-05-23

Handle 2FA errors on all commands
Fix bug with CentOS 6

3.8.0 2014-05-20

Remove 2fa:enable/disable, please visit https://dashboard.heroku.com for these

3.7.5 2014-05-19

Keep using the system json gem when possible

3.7.4 2014-05-19

Fix issue with json dependency in ps commands

[swills]

Approved