Added check to ensure that period is found in version string of RSS node

Came across an RSS feed in the wild that did not contain a version attribute in the RSS node. This triggered a panic due to a bounds error on the slice, as strings.Index() was returning -1 on failure to locate the position of the period. 

The fix was a simple check to ensure that strings.Index() had indeed returned a value that was not -1, and to set the default values for major and minor to zero, so that feed.testVersion() would trap it as usual.