remove callside @inline as this requires 1.8

[milankl]

fixes #81

@inline doesn't seem to be required for no allocations

julia> A = rand(OctaHEALPixGrid,16);
julia> B = rand(OctaHEALPixGrid,16);
julia> @btime RingGrids.eachgridpoint($A,$B)
  2.535 ns (0 allocations: 0 bytes)
Base.OneTo(1024)