RFC: Freeze METADATA for package versions that support Julia 0.3

[tkelman]

In the interest of moving nightly PackageEvaluator testing to running against 0.4, 0.5, and 0.6-dev, I'm proposing we freeze METADATA for Julia 0.3. New package versions that support Julia 0.3 would fail the Travis check, by default. We can make case-by-case exceptions if absolutely needed, but I believe this is the safest path forward to leaving Julia 0.3 alone - what currently works should remain working, and nothing new could break by releasing some new package versions that do support 0.3 when others that may depend on that package have moved on to only supporting Julia 0.4 a while ago.

I'll leave this open for several days in case anyone objects, and send an announcement out to julia-users (https://groups.google.com/forum/#!topic/julia-users/SRJqJKDmxGI). If you're a package author, check the minimum Julia version dependency in your REQUIRE file. If it already says 0.4 or later, you don't need to do anything. If it still says 0.3, this change would mean you should raise the minimum Julia version to (at least) 0.4 before making your next tag. And when you make that new tag, since it's dropping Julia 0.3 support it should use a new package minor version via Pkg.tag("Foo", :minor).