Do not show the Turbo link in the header if Gears is already being used, props...

Do not show the Turbo link in the header if Gears is already being used, props markjaquith, fixes #8276

git-svn-id: https://core.svn.wordpress.org/trunk@10031 1a063a9b-81f0-0310-95a4-ce76da25c4cd