Fixes readings error

I was having a similar error to [issue 1](https://github.com/practicalarduino/SHT1x/issues/1) and [issue 3](https://github.com/practicalarduino/SHT1x/issues/3)

I noticed that the most significant byte was being lost. I changed the data type returned by shiftIn and that appears to resolve the issue (on my end at least).