This reverts commit c05d24c5, reversing
changes made to 66c40d0e.

Travis-ci changed the default build environment to Xenial as explained in https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment
This causes builds meant for Debian Jessie to break as noted by @wRAR in https://github.com/scrapy/scrapy/issues/3917#issuecomment-516426389

This change pins the environment to known working ubuntu trusty distribution prior to dropping Jessie support and upgrade to Xenial as base.

Closes #1369

Added constrain on lxml version to avoid error when using Python 3.4 (#3912)

Changes and release notes for 1.7.2

add instructions about how to define output file when running scrapy …

* Update statscollectors.py

* Update statscollectors.py

* Update statscollectors.py

* Update crawler.py

* Update crawler.py

* corrected tests

* Update test_utils_log.py

* Update crawler.py

* Update crawler.py

* interchanged order

* correced

Deprecate scrapy.utils.datatypes.MergeDict in favor of collections.ChainMap

Prevent deprecation warnings

* assertRegexpMatches -> assertRegex

* assertRegexpMatches -> assertRegex
* invalid escape sequence \[

Remove docs/topics/ubuntu.rst

Extend the topic about broad crawls with information about memory usage

Added section on how to treat memory consumption problems of broad crawls.