Highlight time if not synchronized

[fhunleth]

An invalid time can cause SSL certificates to report that they're expired. The reason is that the system time is before the certificate is valid. As you'd expect, the word "expired" makes you think that something else is wrong. While the docs need to be improved to debug this situation, I think that it may be helpful to see the system time render in orange (not red, since it's not necessarily wrong) if NervesTime.synchronized? is false.