Test SSH in travis

Set up the ssh credentials so we are able to talk to localhost and
issue git commands. Move to use a script, as the command list is
getting somewhat long.

While here, delay installing valgrind until we need it, as it and its
dependencies are by far the largest downloads and this allows us to
start compiling (and failing) faster and we only incur this cost when
the test suite runs successfully.
remote: relax the url rules

Accept any value for the remote's url, including an empty string which
we used to reject as invalid configuration.

This is not quite what git does (although it has its own problems with
such configurations) and it makes it harder to fix the issue, by not
letting the user modify it.

As we already need to check for a valid URL when we try to connect to
the network, let that perform the check, as we don't need to do it
anywhere else.
Merge pull request #1774 from libgit2/test-ssh

Test SSH in travis
Merge pull request #1783 from libgit2/cmn/relax-remote

remote: relax the url rules