Fixed solidity version in contracts impedes proper integration

[miguelmtzinf]

Some developers have been raising issues in regards to the solidity version used in the contracts. Fixed versions makes integrations tough, due to how Solidity and current tools work.

PR #762 softened the solidity version of the contracts susceptible to be integrated with. However, integrations are happening with more contracts and it makes sense to remove the fixed version and even use ^.