Upgrade Twig to 1.18.1 from 1.18.0

Follow-up to #2363741: Upgrade Twig to 1.16.* from 1.15.*

Follow-up to #2172235: Upgrade Twig to 1.15.* from 1.12.*

Problem/Motivation

Change Log Diff:

* 1.18.0 (2015-01-25)
* fixed some error messages where the line was wrong (unknown variables or argument names)
* added a new way to customize the main Module node (via empty nodes)
* added Twig_Environment::createTemplate() to create a template from a string
* added a profiler
* fixed filesystem loader cache when different file paths are used for the same template

* 1.17.0 (2015-01-14)
* added a 'filename' autoescaping strategy, which dynamically chooses the
autoescaping strategy for a template based on template file extension.

* 1.16.3 (2014-12-25)
* fixed regression for dynamic parent templates
* fixed cache management with statcache
* fixed a regression in the slice filter

Proposed resolution

Update Twig to 1.18.* from 1.16.*

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A