rails3: updated .gitignore

Change-Id: I2af3335578140c943b962dee7fa506bba62948d3
Reviewed-on: https://gerrit.instructure.com/32171
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>