use `request.session.id` instead of `request.session_options[:id]`

for rack >= 1.5

`request.session_options` does not contains `:id` key.

@see https://github.com/rack/rack/commit/83a270d648205302edbf4534fd3a9055cb121282

remove `rescue session.session_id` because it was written for compatibility 5 years ago.
@see https://github.com/jpmobile/jpmobile/commit/41aeb7bc103dfefd7ceac7396c01dac4b882f367
Merge pull request #67 from drecom/fix_jpmobile_session_id_for_rack_1_5

use `request.session.id` instead of `request.session_options[:id]`