add application/gzip mime type

on ubuntu 14.04, the tarred file referenced in
spec/models/content_migration_spec.rb:370
returns "application/gzip" as its mime type
which is not in the current list of mime types.

fixes SD-1229

test plan:
* specs pass

Change-Id: I0575393e60da83ba7089a0acede18eaf4a6c1db8
Reviewed-on: https://gerrit.instructure.com/87794
Tested-by: Jenkins
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>

Author: Landon Wilkins

gems/canvas_mimetype_fu/lib/canvas_mimetype_fu/mime_types.yml

@@ -189,6 +189,7 @@ gsf: application/x-font-type1
 gtar: application/x-tar
 gvp: text/x-google-video-pointer
 gz: application/x-gzip
+gzip: application/gzip
 h: text/x-chdr
 h++: text/x-c++hdr
 hbc: application/vnd.hbci

gems/canvas_unzip/lib/canvas_unzip.rb

@@ -104,7 +104,7 @@ def self.each_entry(archive_filename)
     file = File.open(archive_filename)
     mime_type = File.mime_type?(file)
 
-    if mime_type == 'application/x-gzip'
+    if ['application/x-gzip', 'application/gzip'].include? mime_type
       file = Zlib::GzipReader.new(file)
       mime_type = 'application/x-tar' # it may not actually be a tar though, so rescue if there's a problem
     end