Update the ordering algorithm to semver 2.0.0.

Note that Version's `le` is not "less than or equal to" now, since `lt`
ignores build metadata. I think the new ordering algorithm satisfies
strict weak ordering which C++ STL requires, instead of strict total
ordering.
Add more tests for build metadata.
auto merge of #7726 : omasanori/rust/semver-2.0.0, r=graydon

The Ord impl of Version refered to the algorithm in release candidate versions of semver. [Semantic Versioning 2.0.0](http://semver.org/spec/v2.0.0.html) says, "Build metadata SHOULD be ignored when determining version precedence."

Note that Version's `le` is not "less than or equal to" now, since `lt` ignores build metadata. I think the new ordering algorithm satisfies strict weak ordering which C++ STL requires, instead of strict total ordering.

BTW, is `a || b || ... || x`-style code better or idiomatic in Rust than `if a { return true; } if b { return true; } ... if x { return true; } return false;`-style one?