[#1817] Upgrade SQLAlchemy to 0.9.x

No changes needed in the CKAN code base, but it requires changes on vdm,
see this branch:

https://github.com/okfn/vdm/tree/sqlalchemy-0.9-support

Also upgraded sqlalchemy-migrate.
[#1817] Update requirements file now that vdm is on pypi
[#1921] implement resource_show with package_show
rebuild search indexes after tracking update
pass start_date as parameter to execute()
Merge branch '1921-resource-show-via-package-show'
Merge remote-tracking branch 'fuhuxia/rebuild-index-after-tracking-update'
Fix typo in docs differemt -> different
Remove method redefinition
Merge branch 'master' into 1817-upgrade-sqlalchemy

Conflicts:
	requirements.in
	requirements.txt
Merge branch '1817-upgrade-sqlalchemy'
Merge branch 'plugins-method-redefinition'