Use the https not git@ protocol to get the RMG-database in .travis.yml

The builds got this far then stopped for 10 minutes and were killed:
$ git clone git@github.com:GreenGroup/RMG-database.git
Cloning into 'RMG-database'...
The authenticity of host 'github.com (192.30.252.128)' can't be established.
RSA key fingerprint is 16:27:ac:a5:76:28:2d:36:63:1b:56:4d:eb:df:a6:48.
Are you sure you want to continue connecting (yes/no)? 
Hopefully the https version can continue without the RSA fingerprint check…


For some reason Github think my rwest/RMG-Py is no longer the same project as GreenGroup/RMG-Py and I cannot send pull requests. While I wait for them to fix this, I have pushed this topic branch to GreenGroup and am sending the pull request from there.