add "hidden-xs" class to logo …

[DivadNojnarg]

Consider the following example with the current version of shinydashboard, on a cellphone for instance:

library(shiny)
library(shinydashboard)
shinyApp(
  ui = dashboardPage(
    dashboardHeader(),
    dashboardSidebar(),
    dashboardBody(),
    title = "Dashboard example"
  ),
  server = function(input, output) { }
)

We have a blue bar above the sidebar trigger taking unnecessary vertical space.

This simple PR aims at adding the extra class "hidden-xs" to "logo", so that it is not displayed on small screen devices when the title is NULL in dashboardheader (even though it is better to give a title):

span(class = if (is.null(title)) "logo hidden-xs" else "logo", title)

[DivadNojnarg]

On the AppVeyor build, I see installation of package 'later' had non-zero exit status

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