Unlink old keg when upgrading dependencies

Installing a formula via `brew install` may trigger upgrades of its
dependencies if the dependencies are installed but outdated. However, we
never unlinked the existing keg in FormulaInstaller#install_dependency
which caused the link step to fail.

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

Library/Homebrew/formula.rb

@@ -156,6 +156,11 @@ class Formula
    ARGV.formulae.include? self
   end
 
+  def linked_keg
+    keg = Pathname.new(HOMEBREW_REPOSITORY/"Library/LinkedKegs"/@name)
+    if keg.exist? then Keg.new(keg.realpath) else nil end
+  end
+
   def installed_prefix
     head_prefix = HOMEBREW_CELLAR+@name+'HEAD'
     if @version == 'HEAD' || head_prefix.directory?

Library/Homebrew/formula_installer.rb

@@ -67,6 +67,7 @@ class FormulaInstaller
     fi.show_header = false
     oh1 "Installing #{f} dependency: #{dep}"
     fi.install
+    dep.linked_keg.unlink if dep.linked_keg
     fi.caveats
     fi.finish
   end