Avoid errors from listen trying to write to read-only directories.

lib/sass/plugin/compiler.rb

@@ -226,6 +226,11 @@ def watch(individual_files = [])
       end
       directories = remove_redundant_directories(directories)
 
+      # A Listen version prior to 2.0 will write a test file to a directory to
+      # see if a watcher supports watching that directory. That breaks horribly
+      # on read-only directories, so we filter those out.
+      directories.reject {|d| File.writable?(d)} unless Sass::Util.listen_geq_2?
+
       # TODO: Keep better track of what depends on what
       # so we don't have to run a global update every time anything changes.
       listener_args = directories + [{:relative_paths => false}]