FormulaInstaller: disambiguate show_header usage

show_header should default to false (we don't want to display it in the
case of installing a single formula without dependencies), but it only
worked this way by accident.

The assignment "show_header = true" creates a local variable named
show_header, and in the case where needed_deps is empty and this
assignment is not actually executed, the latter usage evaluates the
local variable instead of calling the method.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula_installer.rb

@@ -15,7 +15,7 @@ class FormulaInstaller
   def initialize ff, tab=nil
     @f = ff
     @tab = tab
-    @show_header = true
+    @show_header = false
     @ignore_deps = ARGV.ignore_deps? || ARGV.interactive?
     @install_bottle = install_bottle? ff
 
@@ -99,7 +99,7 @@ class FormulaInstaller
           end
         end
         # now show header as all the deps stuff has clouded the original issue
-        show_header = true
+        @show_header = true
       end
     end