Fix errors on julia 0.7, drop 0.5 support

[carlobaldassi]

All tests pass (locally) without deprecation warnings on both julia 0.6 and 0.7 (some warnings still remain, e.g. the one about exporting ascii).

This supersedes #21. I've seen that in that PR there was some debate on whether to abandon the package, but it is required in particular by JLD, so I've put in the work to fix it. Hopefully, no more major fixes will be required after 1.0.

[carlobaldassi]

@stevengj thanks for reviewing. If there are no objections/comments from others this may be merged then? (I don't have write access in this repo.)

[ararslan]

This seems fine to me, but I kind of assumed that use of this package would have died out by the time people started using Julia 0.7. (I actually thought this repo was in JuliaArchive but I guess it got moved for some reason?)

[carlobaldassi]

It's used by JLD, which aims to support archives from older julia versions too.

[ararslan]

I also hoped people would move from JLD to JLD2. 😄

[carlobaldassi]

Sure. But what if I have an old archive written by JLD? (I certainly have a few of those around...)
Also, the JLD2 readme still has this disclaimer: "If your tolerance for data loss is low, JLD may be a better choice at this time."
Anyway, I don't see any reason not to fix the package once the work to fix it is done, right?

[ararslan]

No, like I said this seems fine to me, I was just surprised someone cared about fixing it. I'd merge this but I don't have commit access in JuliaStrings (though I could have merged it before the repo was moved from JuliaArchive).