fix docs.rs build for gdal-sys (hopefully)

[michaelkirk]

The gdal-sys docs are currently failing to build:
https://docs.rs/crate/gdal-sys/0.3.0/builds/326757

I think this is because the build script requires libgdal to actually be installed. This adds a short circuit to skip the normal build process and hardcodes a version of the prebuilt bindings to use when generating docs.

[michaelkirk]

travis-ci build failure is same as the build failure in master:

error: no matching package named `gdal-sys` found
location searched: /home/travis/build/michaelkirk/gdal/gdal-sys
prerelease package needs to be specified explicitly
gdal-sys = { version = "0.3.1-alpha.0" }

I think it's because we're using a semantic qualifier in the constraints, which isn't compatible with "alpha" (and other prerelease) version strings.

[jdroenner]

Ah yes, i guess we should remove the alpha. Cargo Release did that. Maybe we could also remove the Version from the gdal-sys dependancy.

[michaelkirk]

Ah yes, i guess we should remove the alpha. Cargo Release did that. Maybe we could also remove the Version from the gdal-sys dependancy.

I'm not familiar with cargo release, but yeah I was surprised by the "alpha" tag.