don't use git to build vendored gem specification file list

fixes CNVS-11072

test plan: in a canvas repository without a .git directory (like a deploy) call
a bundle command (like script/console).  it should not give you warnings about
missing .git directories

Change-Id: I2daa1371d97c94f7c3db81d3fd2bad387fcabaf8
Reviewed-on: https://gerrit.instructure.com/29947
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>