Pin build environment to Trusty for Jessie builds

[dangra]

Fixes #3917

[codecov]

Codecov Report

Merging #3922 into 1.7 will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              1.7    #3922      +/-   ##
==========================================
+ Coverage   85.01%   85.56%   +0.55%     
==========================================
  Files         164      164              
  Lines        9567     9567              
  Branches     1436     1436              
==========================================
+ Hits         8133     8186      +53     
+ Misses       1180     1133      -47     
+ Partials      254      248       -6

Impacted Files	Coverage Δ
scrapy/pipelines/files.py	66.53% <0%> (+1.15%)	⬆️
scrapy/core/scraper.py	88.51% <0%> (+2.02%)	⬆️
scrapy/core/downloader/handlers/http11.py	92.53% <0%> (+2.61%)	⬆️
scrapy/extensions/feedexport.py	83.48% <0%> (+5.04%)	⬆️
scrapy/core/downloader/tls.py	90% <0%> (+12.5%)	⬆️
scrapy/utils/boto.py	73.33% <0%> (+26.66%)	⬆️
scrapy/core/downloader/handlers/s3.py	95.16% <0%> (+32.25%)	⬆️

[dangra]

After thinking it a bit more I prefer #3923 approach