LeftAndMain's init() method calls two Versioned methods and this makes it impossible to have the Silverstripe-admin module installed without the Versioned module.

In other parts of the Silverstripe-admin code we have the verification of the presence or absence of the Versioned class, instead in the init() method it is missing.

Please, could you also introduce in the init() method the verification of the presence of the Versioned module in order to install the administrative panel without the Versioned module?

Like in the file commit, for example.

indentation correction for travis