omego install

As pointed out by @pwalczysko the current behaviour of  `omego upgrade` automatically creating an `OMERO-CURRENT` symlink if it doesn't exist is annoying if you inadvertently run omego from the wrong directory. This PR just moves that behaviour into a new `omego install` command (optionally with `--initdb`).

`install` and `upgrade` should also check straight away for the presence/absence of the `OMERO-CURRENT` symlink

Other than this `omero install` is identical from the previous `omego upgrade` behaviour, i.e. not yet a proper installer.

In addition travis has been split into two sub-builds, the old wget+upgrade and a new install, though the wget could probably be replaced with install.