Replaced type checks with Solium util functions

[Cisplatin]

There's currently the following issue happening on testing:

TypeError: source.isAssignment is not a function

So it seems that context.getSourceCode() does not have the functions that were specified in the Solium utils file. Not sure if this is a result of the funky testing stuff happening for this repo or if I've done something wrong. If someone could help point out the issue that'd be greatly appreciated.

[duaraghav8]

(reproducing my reply from discord)

Can you download the latest version (1.0.8) and then lint with this plugin? The latest contains the utils you pushed
The hack we used to fix the npm test will only cause the tests to break. But manual linting over contracts should work because you're using 1.0.8 for it.

I'm working on a permanent (and clean) fix for the testing problem, will merge your PR once that is resolved, apologies for the wait!