Add architecture ppc64le to travis build

[zazzel]

Add support for architecture ppc64le.
This is part of the Ubuntu distribution for ppc64le. This helps us simplify testing later when distributions are re-building and re-releasing.
For more info tag @gerrith3
Change the distro to Xenial because the package requires python >= 3.5.

[codecov]

Codecov Report

Merging #93 (a4b9959) into master (3912783) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   39.75%   39.78%   +0.02%     
==========================================
  Files          39       39              
  Lines        6862     6857       -5     
  Branches     1229     1227       -2     
==========================================
  Hits         2728     2728              
- Misses       3884     3885       +1     
+ Partials      250      244       -6     

Impacted Files	Coverage Δ
lttnganalyses/cli/io.py	65.89% <0.00%> (ø)
lttnganalyses/cli/irq.py	60.46% <0.00%> (ø)
lttnganalyses/cli/sched.py	0.00% <0.00%> (ø)
lttnganalyses/cli/memtop.py	0.00% <0.00%> (ø)
lttnganalyses/cli/periods.py	0.00% <0.00%> (ø)
lttnganalyses/core/period.py	43.30% <0.00%> (ø)
lttnganalyses/cli/command.py	42.40% <0.00%> (+0.38%)	⬆️

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