Spent a fun morning figuring out WTF was going on here…
Take the following extract from a Travis.yml file:
matrix:
include:
#Nightly builds for upload. Use lowest possible Mono version for compatibility purposes
- os: linux
dist: trusty
mono: 5.20.0
Here, I don’t think I’m doing anything too interesting at all. I want to build against the previous stable version of Mono. ( Mono Releases | Mono )
This though throws a wobbly from this sample job-
with this error:
Installing Mono
E: Unable to locate package nuget
E: Unable to locate package referenceassemblies-pcl
It took me some experimentation to figure out that Mono 5.20.0 doesn’t actually appear to be available.
Without access to the apt configuration, I’m not sure entirely why, as it’s unclear where exactly you’re pulling the packages from.
However, altering the Mono version to 5.20.1 (latest found when digging through the Github releases here: Releases · mono/mono · GitHub ) then works OK.
I also tried 5.20 as the release string, but this gave me 5.2 (Didn’t try 5.20.* , unsure if this should work)
Travis really ought to be either loading the most current release in the 5.20 series gracefully (as the last number is just the point release), or at the least giving me a more sensible error message 
cc @akoeplinger @nterry (Unable to mention the third person as this forum won’t let me…)