ci: remove accidental distro upgrade

[bruce-one]

Alternative to #1165 as Trusty seems to be the lowest release supported by the official Node build.

[bruce-one]

@kewde Just an fyi; this did pass :-)

Apologies for bumping all the way to Xenial originally (rather than Trusty) :-(

Trusty looks to be compatible with RHEL7 (as per #1164):

# strings /usr/lib/x86_64-linux-gnu/libstdc++.so.6 | grep GLIBCXX
GLIBCXX_3.4
GLIBCXX_3.4.1
GLIBCXX_3.4.2
GLIBCXX_3.4.3
GLIBCXX_3.4.4
GLIBCXX_3.4.5
GLIBCXX_3.4.6
GLIBCXX_3.4.7
GLIBCXX_3.4.8
GLIBCXX_3.4.9
GLIBCXX_3.4.10
GLIBCXX_3.4.11
GLIBCXX_3.4.12
GLIBCXX_3.4.13
GLIBCXX_3.4.14
GLIBCXX_3.4.15
GLIBCXX_3.4.16
GLIBCXX_3.4.17
GLIBCXX_3.4.18
GLIBCXX_3.4.19

[freedbrt]

Will it be merged ? I have the same problem, can't run with latest centos 7.