Update for Julia 0.7

[wookay]

• drop support Julia 0.6
• import JLD2 (though not used currently but https://github.com/JuliaDSP/MFCC.jl/issues/11 )
• ceil(Int,) for https://github.com/JuliaDSP/MFCC.jl/pull/10

[davidavdav]

Hi thanks for these efforts. Please be aware there seems to be a similar effort in #13, at some stage I have to choose.