clean up iOS versioning in scripts

[incanus]

Too many version determinations!

• mapbox-gl-native/scripts/ios/package.sh

  Line 124 in c35f18f

  DOCS_VERSION=$( git tag | grep ^ios | sed 's/^ios-//' | sort -r | grep -v '\-rc.' | grep -v '\-pre.' | sed -n '1p' | sed 's/^v//' )

• mapbox-gl-native/ios/docs/install_docs.sh

  Line 10 in c35f18f

  DOCS_VERSION=$( git tag | grep ^ios | sed 's/^ios-//' | sort -r | grep -v '\-rc.' | grep -v '\-pre.' | sed -n '1p' | sed 's/^v//' )

• https://github.com/mapbox/mapbox-gl-native/blob/5d56131eb48dfd1adc1135a8d061ee3bed8c5802/scripts/ios/framework.sh#L29

[incanus]

This should also expose the option to tack on any -pre.X or -rc.Y designation, if desired.

[1ec5]

Not critical for 3.0.0.

[incanus]

Also useful here is git tag --sort=-v:refname to combat the semantic versioning problem, but that still puts something like ios-v3.0.0-pre.10 ahead of ios-v3.0.0 final.

[incanus]

We may want to grab something like node-semver as a packaging dependency rather than reinvent the wheel for version comparisons.

[1ec5]

Not critical for 3.1.0.

[1ec5]

We still have a bunch of version-calculating code scattered about, but now it’s all based on git describe instead of fragile sorting. I think we’re good here.