Removed Ubuntu 14.04 and 15.10 CI and pre-built package support

Hi all,

Per our plan for handling Ubuntu releases, we have now shut down the Ubuntu
14.04 and Ubuntu 15.10 CI. At this point we are running CI and building
pre-built packages for Ubuntu 16.04 x86_64 and Ubuntu 16.10 x86_64.

See the following email for more details on our plan for handling adoption
of newer Ubuntu releases:

Thank you!

Todd Fiala