Revise/add rake tasks for search index mgmt.  Bump Tire to 0.6.2.

Tire is being upgraded in order to get the latest import-related fixes, such as:

https://github.com/karmi/retire/commit/13abc5b351a66301e246adf9e657fd3b5e9c66aa
https://github.com/karmi/retire/commit/45eb4f988ddc51bd51236e0c918804dc3ee29a9b
https://github.com/karmi/retire/commit/7117550ac76f849dd8c55cb1f2bf24a68de40d7c
https://github.com/karmi/retire/commit/67b89035cf7e63b87f6656eda6e73be500f4977a
Merge pull request #104 from edx/jsa/index-migr2

Revise/add rake tasks for search index mgmt.  Bump Tire to 0.6.2.