Allow Truffle V4 style solc config

[cgewecke]

#434

Handles this case, coercing V4 format optimization to false:

module.exports = {
  plugins: ["solidity-coverage"],

  compilers: {
    solc: {
      version: "0.5.11"
    }
  },
  solc: {
    optimizer: {
      enabled: true,
      runs: 200
    }
  }
};

[codecov-io]

Codecov Report

Merging #449 into buidler-plugin will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           buidler-plugin     #449      +/-   ##
==================================================
+ Coverage           99.22%   99.22%   +<.01%     
==================================================
  Files                  20       20              
  Lines                 776      778       +2     
==================================================
+ Hits                  770      772       +2     
  Misses                  6        6

Impacted Files	Coverage Δ
plugins/resources/truffle.utils.js	100% <100%> (ø)	⬆️

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