Update rebar.config

I've encountered an error while running chef-server cookbook:

```
Pulling envy from {git,"git@github.com:manderson26/envy.git",
                       {branch,"master"}}
Cloning into 'envy'...
Host key verification failed.
fatal: The remote end hung up unexpectedly
ERROR: git clone -n git@github.com:manderson26/envy.git envy failed with error: 128 and output:
Cloning into 'envy'...
Host key verification failed.
fatal: The remote end hung up unexpectedly
```

Turned ':' to '/' in envy.git path