Add Ubuntu 18.04 LTS as a target test platform for Travis CI

[PeterHamilton]

This change adds Ubuntu 18.04 LTS (Bionic Beaver) as a target test platform in the Travis CI configuration file. New test builds for all unit, integration, functional, style, security, and doc checks should now be built for Bionic during continuous integration runs.

[codecov-io]

Codecov Report

Merging #584 into master will decrease coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #584      +/-   ##
==========================================
- Coverage   96.58%   96.58%   -0.01%     
==========================================
  Files          79       79              
  Lines       16887    16887              
==========================================
- Hits        16311    16310       -1     
- Misses        576      577       +1

Impacted Files	Coverage Δ
kmip/core/primitives.py	97.03% <0%> (-0.19%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update ae05118...00179ee. Read the comment docs.