C#: install PCL reference assemblies package

This is required to build portable class libraries.
Update tests
Merge pull request #361 from akoeplinger/patch-1

C#: install PCL reference assemblies package
Bypass rbx dependency install checks

This fixes https://github.com/travis-ci/travis-ci/issues/3056
Merge pull request #362 from travis-ci/ha-bug-rbx-on-container

Bypass rbx dependency install checks
List localhost first in /etc/hosts for 127.0.0.1 and ::1

This fixes https://github.com/travis-ci/travis-ci/issues/3054
Revert "List localhost first in /etc/hosts for 127.0.0.1 and ::1"

This reverts commit 95d3be1f4f63ad6e9c9a0669e77d3ba23be4b20f.
Add appliance to reorder aliases for 127.0.0.1 in /etc/hosts

Redo fix for https://github.com/travis-ci/travis-ci/issues/3054
Send STDERR localhost fix to /dev/null

It litters the build log due to https://github.com/travis-ci/travis-ci/issues/2969 and most likely cause some unnecessary alarm.