Add newline for append_file arguments in add_javascripts (install)

Using append_file without a new line in add_javascripts() causes the
file `all.js' to be saved without a new line at the end and making the
next (different) plugin install fail to write valid data to this file.
For example after installing spree_reviews which appends jquery.rating
to all.js the diff looks like:

   -//= require store/spree_i18n
   \ No newline at end of file
   +//= require store/spree_i18n//= require jquery.rating

Added '\n' suffix for the string arguments of append_file to fix.

Signed-off-by: Ali Polatel <ali.polatel@ozguryazilim.com.tr>