Allow "devel-only" formulae

It seems only natural that this should be possible, or at the very
least, it should not result in calling methods on nil.

Library/Homebrew/formula.rb

@@ -73,7 +73,8 @@ def determine_active_spec
     when @head && ARGV.build_head?        then @head    # --HEAD
     when @devel && ARGV.build_devel?      then @devel   # --devel
     when @bottle && install_bottle?(self) then @bottle  # bottle available
-    when @stable.nil? && @head            then @head    # head-only
+    when @devel && @stable.nil?           then @devel   # devel-only
+    when @head && @stable.nil?            then @head    # head-only
     else @stable
     end
   end

Library/Homebrew/test/test_formula_validation.rb

@@ -59,6 +59,17 @@ def initialize; end
     end
   end
 
+  def test_devel_only_valid
+    assert_nothing_raised do
+      formula do
+        devel do
+          url "foo"
+          version "1.0"
+        end
+      end
+    end
+  end
+
   def test_head_only_valid
     assert_nothing_raised do
       formula do