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/requirements.txt b/requirements.txt
index bfdbd62..fb94472 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 pyscard
 pyserial
 pytlv
-cmd2==1.5
+cmd2>=1.5
 jsonpath-ng
 construct>=2.9.51
 bidict