Use cymruwhois from pypi and remove submodule

- cymruwhois maintainer recovered his access to pypi and
  uploaded latest package. There is no need to use it as
  submodule anymore. So we added an external dependency.

- Refactor whois data caching in dnstraceroute and unbreak
  caching mechanism which was broken since previous commit
  due to a bug in time delta calculation.

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "cymruwhois"]
-	path = cymruwhois
-	url = https://github.com/JustinAzoff/python-cymruwhois.git