Change build matrix to use go 1.11 and 1.12
Change build matrix to use go 1.11 and 1.12

Signed-off-by: Craig Rodrigues <c4b5a8c0fafd56ae12499ba1d5d55022dbe4108e@portworx.com>

libopenstorage/openstorage,libopenstorage/openstorage,libopenstorage/openstorage