fix clone_repos and web endpoint

clone_repos calls `self.execute(command)`, but this method does not exist.

See a half fix: https://github.com/iit-cs579/teachers_pet/commit/71af808b530ca693efdf6a56694c0300a83f2a00

However, this still does not fix the problem, as I get:
```
Cloning assignment repositories for students...
 --> Cloning: 'git clone https://www.github.com/iit-cs579/ayacha-asg.git'
Cloning into 'ayacha-asg'...
Username for 'https://www.github.com':
```

It appears that the endpoint is wrong (https://www.github.com should be https://github.com ). See the fix: https://github.com/iit-cs579/teachers_pet/commit/8a5137414fcff0f7448f6c3f658fb49e9f89760a

Thanks for a great library -- it's much better than the glue and duct tape I used last semester!