Bundler: Set local deployment via config, not command flag

Fixes the following deprecation:

[DEPRECATED] The `--deployment` flag is deprecated because it relies on
being remembered across bundler invocations, which bundler will no
longer do in future versions. Instead please use `bundle config set
--local deployment 'true'`, and stop using this flag

bin/post_deploy.bash

@@ -1,6 +1,7 @@
 cd "$(dirname $BASH_SOURCE)"/..
 
-bundle install --deployment --binstubs ../gem-bin
+bundle config set --local deployment 'true'
+bundle install --binstubs ../gem-bin
 cd $1
 ../../gem-bin/sass --update assets/sass:assets/css --style compressed
 ../../gem-bin/jekyll build