Fix context.StatementRef

Resolves #91

Supersedes https://github.com/RusticiSoftware/TinCanJS/pull/94
I think this change is unnecessary, the default is to return StatementRef since that is the value in the object itself. If it isn't then the value for 0.95 and non-0.9 will be the same, so if you were intending to make it something else I don't believe you have. I've included backing it out in my latest commit, so if it is necessary (with change) then we'll need to add it back in.