fix mimetype_fu gemspec

Change-Id: I6123bf65c43fd0ed51a3365fee6815fafab02b48
Reviewed-on: https://gerrit.instructure.com/29498
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>