travis: try fixing Python 3.7 installation
travis: try fixing Python 3.7 installation

mistio/libcloud,mistio/libcloud,Scalr/libcloud,Kami/libcloud,mistio/libcloud,apache/libcloud,ByteInternet/libcloud,Scalr/libcloud,Kami/libcloud,Kami/libcloud,ByteInternet/libcloud,ByteInternet/libcloud,apache/libcloud,Scalr/libcloud,andrewsomething/libcloud,andrewsomething/libcloud,andrewsomething/libcloud,apache/libcloud