Add builder-ethers test to CI

[cgewecke]

Also cleaning up a couple small things...

• Make sure the network requested by --network is printed to terminal correctly
• Only default to config-set ganache when none is provided in the api.ganache invocation.

[codecov-io]

Codecov Report

Merging #441 into buidler-plugin will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           buidler-plugin     #441      +/-   ##
==================================================
+ Coverage           99.21%   99.21%   +<.01%     
==================================================
  Files                  18       18              
  Lines                 766      768       +2     
==================================================
+ Hits                  760      762       +2     
  Misses                  6        6

Impacted Files	Coverage Δ
lib/api.js	100% <100%> (ø)	⬆️
dist/plugin-assets/buidler.utils.js	100% <100%> (ø)	⬆️
dist/buidler.plugin.js	100% <100%> (ø)	⬆️
dist/truffle.plugin.js	100% <100%> (ø)	⬆️

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