travis: add non-default shell testing to travis.
Bugfix: Replace bashisms with standard sh to fix build on non-BASH systems
Bugfix: Replace bashisms with standard sh in gitian descriptors
Bugfix: Replace bashisms with standard sh in tests/tools
Merge pull request #5038

0b17964 Bugfix: Replace bashisms with standard sh in tests/tools (Luke Dashjr)
ab72068 Bugfix: Replace bashisms with standard sh in gitian descriptors (Luke Dashjr)
b77b4ed Bugfix: Replace bashisms with standard sh to fix build on non-BASH systems (Luke Dashjr)
d6b0539 travis: add non-default shell testing to travis. (Cory Fields)