Switch mavis submodule to use HTTPS (#195)

In this pull request I have tried to update ".gitmodules" file to switch
mavis (submodule) from SSH to HTTPS, this is the change which is user
friendly and hassle free cloning for all users, especially for people
those who do not have SSH access and prefer HTTPS for real time
operations.

Updated the above mentioned file ".gitmodules" to use HTTPS for the
mavis submodule:
"https://github.com/sparcians/mavis.git"
Synchronized and reinitialized the mavis submodule to reflect the new
URL.
Verified the submodule is correctly cloned and also functional with new
integration.

This is a enhancement feature which improves accessibility and ease the
process of setting up locally for those w/o SSH credentials.

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "mavis"]
 	path = mavis
-	url = git@github.com:sparcians/mavis
+	url = https://github.com/sparcians/mavis.git
 [submodule "stf_lib"]
 	path = stf_lib
 	url = https://github.com/sparcians/stf_lib.git