Support "pragma experimental"

The Solidity 0.4.17 release notes say:

As we are getting closer to the next breaking release, we want to give everyone a heads up by introducing pragma experimental “v0.5.0” which already enables some of the new safety features of the 0.5.0 release.

Currently I get:

There was a problem instrumenting ./coverageEnv/contracts/ProofOfExistence1.sol: SyntaxError: Expected "solidity", comment, end of line, or whitespace but "e" found. Line: 2, Column: 8
Exiting without generating coverage...

Where line 2 is:

pragma experimental "v0.5.0";

[cgewecke]

@area self-assigning and will try to fix parser today unless you’re already working on this. This and #131 also need:

• bump dev dep to truffle 4 beta
• a simple test that uses the new language features

[elenadimitrova]

Let’s reopen this as truffle 4 beta includes solc@0.4.17 update. It’s back to being a solidity-coverage issue.