Add CI checks for other operating systems

[tomuben]

Background

We discovered that different versions of bash can cause different behavior of exaslct command line tools, see #48 .
Because of that we need at least a CI check for CentOs 7, which comes with bash 4.2 (the minimum compatible version).

Acceptance Criteria

A new CI check which validates export functionality on push, in a CentOS 7 enviornment.