Pkg: need a way for Packages to depend on development versions of julia

[kmsquire]

As noted here: JuliaData/DataFrames.jl#287 (comment)

DataFrames is about to change | to |>, to mimic recent changes in Julia. Unfortunately in doing so, it will make itself incompatible with versions of Julia from a week ago. There are multiple ways to deal with this, but at least one of them should be chosen:

1. Make it very clear that the development branch is unstable, and that things might break there at any time, that it may need to be constantly updated, etc.. At this point in Julia development, a number of people have likely switched to the development branch, but may not have kept up to date.
2. Start tagging subversions of Julia at regular intervals. This would require picking a specific mechanism (e.g., all numbering before a 0.3 release could be 0.2.99.x, where x could be incremented)
3. Allow dependency on julia starting at (or up to) a specific git hash (probably hard to implement).
4. Allow dependency on julia starting at (or up to) a specific date/time.

There are probably others...

( Edit : fixed "pre-" version number)

[johnmyleswhite]

I think the honest thing for us to do until we hit 1.0 is Option 1, although the others could be useful to have available.