Travis: fix shell syntax for version specifier
Travis: fix shell syntax for version specifier

shamangeorge/beets,shamangeorge/beets,sampsyo/beets,beetbox/beets,jackwilsdon/beets,ibmibmibm/beets,ibmibmibm/beets,jackwilsdon/beets,jackwilsdon/beets,SusannaMaria/beets,sampsyo/beets,SusannaMaria/beets,ibmibmibm/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets,sampsyo/beets,jackwilsdon/beets,beetbox/beets,SusannaMaria/beets,shamangeorge/beets,shamangeorge/beets,beetbox/beets,SusannaMaria/beets