source/install: mention Tsuru's PPa
source/install: mention Tsuru's PPa

Related to globocom/tsuru#555.

scorphus/gandalf,gleez/gandalf,marcelometal/gandalf,marcelometal/gandalf,gleez/gandalf,tsuru/gandalf,marcelometal/gandalf,scorphus/gandalf,tsuru/gandalf,flaviamissi/gandalf,gleez/gandalf,flaviamissi/gandalf