Replace isalpha with isletter

[mlhetland]

isalpha is deprecated.

[coveralls]

Coverage increased (+4.2%) to 78.859% when pulling 43c1071 on mlhetland:patch-1 into 48ec088 on dcjones:master.

[sglyon]

Thank you for doing this. I believe that this is a Julia 0.7 only feature. Perhaps there is a corresponding item in Compat.jl that would make this PR work on older versions of Julia also?

[mlhetland]

Yes, Compat.jl adds isletter as an alias for isalpha, so it should still work.

[sglyon]

Ahh yes -- thanks for that. I had forgotten that we did a bare using Comat and that isletter would be brought into scope.

I think we need to set a minimum version bound on compat to ensure that users get a version of Compat that has isletter. Will you please add that to this PR?

[mlhetland]

Updated REQUIRE now. There should be a Project.toml, too, to avoid warnings with 0.7 (and, eventually, problems with 1.x).

[sglyon]

Thank you!

yes, we need to construct a Project.toml for future julia. We will do that soon

I will merge this as is.