Ok I solved the issue. I write the workaround just for information if someone gets into the same problem.
I went to my directory gems under bundler on my ruby version and found out that 4 different directories were created for spree_i18n. I just changed the name of all of them and try again the bundle install from my app directory. Now everything worked properly and the module is added to the bundle.
guillermo@willylaptop:/usr/local/rvm/gems/ruby-2.1.0/bundler/gems$ ll
total 88
drwxrwsr-x 22 guillermo rvm 4096 Feb 6 14:08 ./
drwxrwsr-x 3 guillermo rvm 4096 Jan 27 18:42 ../
drwxrwxr-x 9 guillermo rvm 4096 Feb 8 17:06 better_spree_paypal_express-f385bbe229c9/
drwxrwxr-x 7 guillermo rvm 4096 Feb 8 17:06 deface-607524622705/
drwxrwxr-x 9 guillermo rvm 4096 Feb 8 17:06 spree_auth_devise-f6f91480655b/
drwxrwxr-x 10 guillermo rvm 4096 Feb 8 17:06 spree-b328e2487bfc/
drwxrwxr-x 8 guillermo rvm 4096 Feb 8 17:06 spree_email_to_friend-f6483b4d8dba/
drwxrwxr-x 10 guillermo rvm 4096 Feb 8 17:06 spree_fancy-d6b6cf03b1f6/
drwxrwxr-x 10 guillermo rvm 4096 Jan 30 14:41 spree-ffda7917903a/
drwxrwxr-x 9 guillermo rvm 4096 Jan 30 14:41 spree_gateway-04c51e6bd24e/
drwxrwxr-x 9 guillermo rvm 4096 Jan 31 09:48 spree_gateway-ab4dcb885418/
drwxrwxr-x 10 guillermo rvm 4096 Jan 30 14:09 spree_i18n-2398a7064abe/
drwxrwxr-x 10 guillermo rvm 4096 Jan 31 09:48 spree_i18n-7b1061b06199/
drwxrwxr-x 10 guillermo rvm 4096 Feb 6 14:08 spree_i18n-a2c64740cdc7/
drwxrwxr-x 10 guillermo rvm 4096 Jan 27 20:01 spree_i18n-a5ab8c91b4dd/
drwxrwxr-x 9 guillermo rvm 4096 Jan 29 14:48 spree-product-assembly-30c8ea090e4a/
drwxrwxr-x 9 guillermo rvm 4096 Jan 31 09:48 spree-product-assembly-586ba3923b64/
drwxrwxr-x 9 guillermo rvm 4096 Jan 29 13:27 spree-product-assembly-85cecdfc828d/
drwxrwxr-x 10 guillermo rvm 4096 Feb 8 17:06 spree_reviews-30c7518d65e4/
drwxrwxr-x 8 guillermo rvm 4096 Jan 31 09:48 spree_static_content-5966c16c9ef1/
drwxrwxr-x 8 guillermo rvm 4096 Jan 30 14:05 spree_static_content-6ac885b6f8c2/
drwxrwxr-x 8 guillermo rvm 4096 Feb 8 17:06 spree_wishlist-56f1f230b0bc/
guillermo@willylaptop:/usr/local/rvm/gems/ruby-2.1.0/bundler/gems$ mv spree_i18n-2398a7064abe/ spree_i18n-2398a7064abe.bck
guillermo@willylaptop:/usr/local/rvm/gems/ruby-2.1.0/bundler/gems$ mv spree_i18n-7b1061b06199/ spree_i18n-7b1061b06199.bck
guillermo@willylaptop:/usr/local/rvm/gems/ruby-2.1.0/bundler/gems$ mv spree_i18n-a2c64740cdc7/ spree_i18n-a2c64740cdc7.bck
guillermo@willylaptop:/usr/local/rvm/gems/ruby-2.1.0/bundler/gems$ mv spree_i18n-a5ab8c91b4dd/ spree_i18n-a5ab8c91b4dd-bck