rails3: select() is ignored when combined with includes() in AREL

instead preload the context and just call context.name directly for rails3.
this fixes issues with a couple integration specs that are expecting the
aliased column 'context_name' in select() to be present.

Change-Id: I024edccf31a43552bb5c01e74a2b312a9771f31b
Reviewed-on: https://gerrit.instructure.com/30226
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>