Failing to parse abstract contracts in Solidity 0.6

The parser is not handling the new `abstract` keyword introduced with Solidity 0.6 and is failing with
```
extraneous input 'abstract' expecting {<EOF>, 'pragma', 'import', 'contract', 'interface', 'library'}
```

You can see an example in this contract
https://etherscan.io/address/0x92d861ebf39661f22eaae05fedcbf31989b17e45#code
