0.5 compat

[sjkelly]

Fix some dep warns and test on 0.5. This should be the only issues for running 0.5. I am also seeing significantly better performance margins on 0.5, and I am not sure where it was gained.

julia> Pkg.test("ImmutableArrays")
INFO: Testing ImmutableArrays
ImmutableArray operation speed relative to native Julia arrays:
+ 0.06773972944842568
* 0.10612913453695451

[coveralls]

Coverage remained the same at 96.897% when pulling c51ecc8 on sjk/05compat into 08e1b30 on master.

[sjkelly]

There are warnings during precompile related to JuliaLang/julia#17618, AFAIK. After precompile it runs without warning, so I suspect this will be fixed when backported to release-0.5.