Fix API for Mastodon version (#2590)

Fix #2589

app/views/api/v1/instances/show.rabl

@@ -4,4 +4,4 @@ node(:uri)         { site_hostname }
 node(:title)       { Setting.site_title }
 node(:description) { Setting.site_description }
 node(:email)       { Setting.site_contact_email }
-node(:version)     { Mastodon::Version }
+node(:version)     { Mastodon::Version.to_s }