platform_key for ARM

[visr]

Came across this in GEOSBuilder, building with TARGET=arm-linux-gnueabihf.
https://travis-ci.org/JuliaGeo/GEOSBuilder/jobs/360575777#L507-L516

julia> triplet(BinaryProvider.Linux(:armv7l))
"arm-linux-gnueabihf"

julia> platform_key("arm-linux-gnueabihf")
WARNING: Platform `arm-linux-gnueabihf` is not an officially supported platform
BinaryProvider.UnknownPlatform()

julia> platform_key("armv7l-linux-gnueabihf")
BinaryProvider.Linux(:armv7l, :glibc, :eabihf)

This can be reversed by chaning the dict in platform_key to :armv7l => "arm", but I'm not sure if this is the way to go, or that we need do deal with sub-arch explicitly.

[staticfloat]

Yeah, my bad, I screwed this one up. Fixed in #53

[visr]

That works, thanks!