Revert "Recognize --head as an alias for --HEAD"

This reverts commit c32ab0a35a2dfa9b593f759ef0ae8b7af077b0b7.

There are other hard-coded uses of HEAD, in build_options for instance.
These all need to be fixed before enabling this.

Library/Homebrew/cmd/install.rb

@@ -19,6 +19,10 @@ module Homebrew extend self
       end
     end
 
+    if ARGV.include? '--head'
+      raise "Specify `--HEAD` in uppercase to build from trunk."
+    end
+
     ARGV.named.each do |name|
       # if a formula has been tapped ignore the blacklisting
       if not File.file? HOMEBREW_REPOSITORY/"Library/Formula/#{name}.rb"

Library/Homebrew/extend/ARGV.rb

@@ -111,7 +111,7 @@ module HomebrewArgvExtension
   end
 
   def build_head?
-    include?('--HEAD') || include?('--head')
+    include? '--HEAD'
   end
 
   def build_devel?