Solves #67

Steps to reproduce the issue
*Set 'version_check' => true in config
*Load (any) page in Browser
*Set 'version_check' => false in config

Since ZendDeveloperTools\Listener\ToolbarListener->getLatestVersion() returns
array(true, '') when 'version_check', $latest = '' and 
$partsLatestRelease = explode('.', $latest) doesn't have index 1 defined.

See #67