Regex tweaks for Amazon Silk and Mobile Safari

I noticed that newer versions of Amazon Silk were appearing as Google Chrome, and that Mobile Safari was sometimes dropping minor version numbers for patch numbers (e.g. showing 7.4 instead of 7.0.4 or 7.1.4).  Here are fixes for both, including new tests from my log files.
The existing test case for NetFront fails if the regex for Amazon Silk is moved ahead of the regex for NetFront