Allow removing version number from kernel spec name

[bluesmoon]

It looks like the kernel spec name will always contain the Julia version number and there is no way to override this.

The code in question (deps/kspec.jl) is this:

juliakspec = joinpath(kerneldir(), "$specname-$(VERSION.major).$(VERSION.minor)$debugdesc")

The same is true for display_name:

"display_name" => name * " " * Base.VERSION_STRING * debugdesc,

It would be good if these could be overridden in the call to installkernel so that we can exclude the version number from the spec name and the display name (or use a different version number).

[JamesWrigley]

As of #1137 the display name can be customized.