Update solidity-coverage for fork.

[cgewecke]

This PR is the solidity-coverage complement to #531. It uses a testrpc based on 6.0.1. Opening this just so you know it is available. It's been tested by bringing #531 into a fork and getting it to run clean there. However, this PR doesn't include any of #531's test fixes.

Probably easier to bump the version yourself when (if) ready because of all the package lock noise. Have followed the procedure outlined in #363. . .but idk.

Some notes:

• solidity-coverage is currently trapped at pragma 0.4.17 and below.
• view and constant work (with warnings). pure compiles but returns a transaction object rather than a value (so, not useful).

Please let me know if you run into any problems, happy to debug and get this working. Also, feel free to close.

Happy fork Zeppelin!
c

[cgewecke]

NB: This PR will need to be followed by another with more extensive changes to the tests when you move above pragma 0.4.17. We've now released a version that can accommodate the new restrictions on view and pure methods as long as they are explicitly invoked using the .call post-fix. A PR for this would touch half (maybe more) of the files in the project so might be good to co-ordinate if / when you are ready. For more info see issue 146 at solidity-coverage.

[cgewecke]

Closing per #573 etc.