Remove support for deprecated password-hashing algorithm

* Remove all methods implementing and checking for the old algorithm
* Document a pre-upgrade step to check for remaining obsolete passwords
* Remove config.salt
Upgrade Rails to 4.1.8 to address CVE-2014-7829
Merge pull request #1736 from matjack1/CVE-2014-7829

Upgrade Rails to 4.1.8 to address CVE-2014-7829
Merge pull request #1733 from dnrce/password-hashing

Remove support for deprecated password hashing algorithm