Upgrade coverage tool

[cag]

Changeset is because solidity-coverage has the following breaking change:

Important: v0.4.0 targets solidity >= pragma 0.4.18 and introduces a breaking change for projects that rely on Truffle's simplified contract method interface. All constant view or pure methods now need to be explicitly invoked using .call. e.g:

[codecov-io]

Codecov Report

Merging #68 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage    91.5%   91.68%   +0.18%     
==========================================
  Files          27       27              
  Lines         671      758      +87     
  Branches      111      111              
==========================================
+ Hits          614      695      +81     
- Misses         57       63       +6

Impacted Files	Coverage Δ
contracts/Events/CategoricalEvent.sol	70% <0%> (-7.78%)	⬇️
contracts/Tokens/StandardToken.sol	71.42% <0%> (-4.58%)	⬇️
contracts/Events/ScalarEvent.sol	79.16% <0%> (-3.45%)	⬇️
contracts/Oracles/DifficultyOracle.sol	61.53% <0%> (-2.1%)	⬇️
contracts/Events/Event.sol	100% <0%> (ø)	⬆️
contracts/Oracles/FutarchyOracle.sol	100% <0%> (ø)	⬆️
contracts/Markets/StandardMarket.sol	100% <0%> (ø)	⬆️
contracts/Oracles/MajorityOracle.sol	100% <0%> (ø)	⬆️
contracts/Oracles/SignedMessageOracle.sol	0% <0%> (ø)	⬆️
contracts/Oracles/UltimateOracle.sol	100% <0%> (ø)	⬆️
... and 3 more

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