cmd2: Constrain version to >= 1.3.0 but < 2.0.0

2.0.0 introduces several incompatible changes, see
https://github.com/python-cmd2/cmd2/blob/master/CHANGELOG.md
as well as https://github.com/python-cmd2/cmd2/issues/1120

As we want to be able to use what distributions ship, let's stay
with 1.x for now.  If piip is used, use 1.5

Change-Id: Iecc953269d5ae9ed9f31b829743c63bdfd29fa61

requirements.txt

@@ -1,7 +1,7 @@
 pyscard
 serial
 pytlv
-cmd2
+cmd2==1.5
 jsonpath-ng
 construct
 bidict