PositionValue lib: restrict to lower than solc version 8

[ewilz]

fixes #198

[moodysalem]

what do we need in 0.7.0? would earlier versions work?

[ewilz]

what do we need in 0.7.0? would earlier versions work?

hmm...well I see now our standard is to do from >=0.5.0 however, I use the TickMath library which uses type(uint256).max which is new in Solidity 0.6.8 even though TickMath also claims to compile from >=0.5.0 But I don't think it will compile lower than 0.6.8, (re PositionValue: unless dependencies are allowed to depend on higher versions?)

[ewilz]

Changed to 0.6.8