Rakefile: handle non-interactive stdin in ask()

STDIN.gets returns nil when stdin is closed/EOF (e.g. after vim/vundle
runs during install), causing `nil.chomp` to abort rake install during
install_term_theme. Return nil from ask() on EOF and have callers bail
out of the iTerm theme prompts gracefully.

Rakefile

@@ -210,7 +210,7 @@ def install_term_theme
   message = "Which theme would you like to apply to your iTerm2 profile?"
   color_scheme = ask message, iTerm_available_themes
 
-  return if color_scheme == 'None'
+  return if color_scheme.nil? || color_scheme == 'None'
 
   color_scheme_file = File.join('iTerm2', "#{color_scheme}.itermcolors")
 
@@ -220,6 +220,8 @@ def install_term_theme
   profiles << 'All'
   selected = ask message, profiles
 
+  return if selected.nil?
+
   if selected == 'All'
     (profiles.size-1).times { |idx| apply_theme_to_iterm_profile_idx idx, color_scheme_file }
   else
@@ -244,7 +246,12 @@ def ask(message, values)
   puts message
   while true
     values.each_with_index { |val, idx| puts " #{idx+1}. #{val}" }
-    selection = STDIN.gets.chomp
+    input = STDIN.gets
+    if input.nil?
+      puts "(no input available — skipping prompt)"
+      return nil
+    end
+    selection = input.chomp
     if (Float(selection)==nil rescue true) || selection.to_i < 0 || selection.to_i > values.size+1
       puts "ERROR: Invalid selection.\n\n"
     else