Make GPv2Authenticator Storage Accessible.

[bh2smith]

Eventually, we may want to include new public view methods to this Authentication contract and should use https://github.com/gnosis/util-contracts/blob/master/contracts/StorageAccessible.sol to do so. Unfortunately, this will require upgrading the util-contracts to be compatible with solc-0.7 beforehand. Thus, this has been extracted out as a separate task from #103.

[bh2smith]

As recently discussed, it may be possible to make the Authenticator compile with a version range so that the existing utils-repo (which is solc-^0.5.2) would still compile together with these contracts.

[bh2smith]

To get the ball rolling on this issue, I have submitted a PR here to util-contracts upgrading the entire repo's solc-compiler to solc v0.6.0. This is a "meet in the middle tactic" that should be least invasive.