Fixes: tool attempting to compile a directory ending in .ts

If you use the [tsd](http://www.tsdpm.com/) to bring in [DefinitelyTyped](https://github.com/DefinitelyTyped) TypeScript definitions, it creates a folder `.d.ts/DefinitelyTyped/<library>/<library.d.ts>` where the root folder `d.ts` was being passed into the `tsc` compiler causing the following error.


    Compiling.
    error TS5012: Cannot read file 'C:/code/grunt-ts/test/DefinitelyTypedTest/d.ts': UNKNOWN, unknown error
    >> Compilation failed