Use HTTPS for the barycorr submodule

This fixes the "Permission denied (publickey)" error:

```
Cloning into '/home/travis/build/vterron/lemon/barycorr'...
Warning: Permanently added the RSA host key for IP address '140.82.112.3' to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.
```

https://stackoverflow.com/q/15674064/184363

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "barycorr"]
 	path = barycorr
-	url = git@github.com:vterron/barycorr.git
+	url = https://github.com/vterron/barycorr.git
 	branch = init