requirements.txt: allow cmd2 versions greater than 1.5

Since we now have fixed the compatibility issues with recent cmd2
versions, we may allow also versions greater than 1.5 in the
requirements.txt

Change-Id: I87702c5250a3660c84458939167bffdca9c06059
diff --git a/setup.py b/setup.py
index 132dfe2..17edad9 100644
--- a/setup.py
+++ b/setup.py
@@ -12,7 +12,7 @@
         "pyscard",
         "serial",
         "pytlv",
-        "cmd2 >= 1.3.0, < 2.0.0",
+        "cmd2 >= 1.5.0",
         "jsonpath-ng",
         "construct >= 2.9.51",
         "bidict",