Code simplification + add pointer check

Code simplification in ResponseWriter

Fix clone method in RounterHandler

Issue #700: fix build

Tell Travis to build on all of its supported CPU architectures.

.travis.yml: Bionic (18.04) doesn't ship g++-9. Unfortunately, although Bionic is old, it's the most recent that Travis CI supports...

.travis.yml: We were building on an OS from 2014 (trusty). Update to more recent that Travis CI supports, 18.04 (Bionic)...

Improve Peer SSL API and remove unnecessary code

Code simplification in Peer class and small fix in Listener

Hide ParserBase internal details

Ran clang-format on all headers (except date.h, since its not ours).

Refactoring in http.h and http.cc

Add override keyword to functions who override