Work around middleman deploy issue #114

Borrowing a hotfix extension from this discussion:
karlfreeman/middleman-deploy#114

The problem: for somereason mm-deploy starts deploying the source files
to gh-pages isntead of the built output. This extension forcibly removes
the build dir every time, which seems to solve the problem.

config.rb

@@ -1,3 +1,5 @@
+require_relative "./extensions/build_cleaner"
+
 activate :directory_indexes
 activate :autoprefixer
 
@@ -18,6 +20,7 @@
 
 configure :build do
   activate :relative_assets
+  activate :build_cleaner
 end
 
 activate :deploy do |deploy|

extensions/build_cleaner.rb

@@ -0,0 +1,12 @@
+#lib/build_cleaner.rb
+
+class BuildCleaner < Middleman::Extension
+
+  def initialize(app, options_hash={}, &block)
+    super
+    FileUtils.rm_rf app.config[:build_dir]
+  end
+
+end
+
+::Middleman::Extensions.register(:build_cleaner, BuildCleaner)