Require Julia >= 1.3

[fingolfin]

... since we are now using a JLL package, which generally requires Julia >= 1.3

Closes #243

Two alternatives come to mind:

1. it would be possible to insert code which can be used in Julia 1.0-1.2 to substitute for the JLLs (see WIP: Pkg Artifacts with legacy mode LoadFlint.jl#4)
2. We could just accept that 4ti2 won't work in older Julia versions, and just disable any code (and tests) using it there.

That said, I think we should just merge this PR, it's the least work, and long term we will want to require the next Julia LTS anyway (right now it looks good for Julia 1.6 becoming that, but time will tell...)

[wbhart]

I don't really see why this works with 1.4. I thought semver would only allow things in the 1.3 series if you annotate it this way. But whatever, it seems to work.

I also thought the Julia people preferred Julia to have a range of supported versions, not just a lower limit. But maybe I got that mixed up too.

[fingolfin]

@wbhart "1.3" is identical to "^1.3" and indicates the version range [1.3, 2). And writing it this way is
pretty much standard, in fact I don't know any Julia package doing it differently.