Replace nan(T) with convert(T, NaN) to stop deprecation warnings in Julia v0.4

Looks like Julia v0.4 will deprecate the nan(T) function in favour of convert(T, NaN). I was trying to get Grid working on v0.4 and changed this to stop the deprecation warnings. 

I couldn't get Grid to pass the rest of its tests on v0.4 because of [#51], which I haven't looked at yet, but on v0.3 everything should be fine.