[WIP] Fall back to 'sort.Slice' on Go 1.7 where 'sort.SliceStable' does not exist

[HaraldNordgren]

Note to self: Please edit CHANGELOG.md and add the change under the appropriate category (NEW FEATURES, IMPROVEMENTS & BUG FIXES) along with the PR number.

fixes #1967

[sdboyer]

The semantics here aren't exactly equivalent, but i have to look more deeply to remind myself of why we used sort.SliceStable() in the first place to be sure of the effects.

Is this actually the only thing that prevents dep from compiling on Go 1.7?

[HaraldNordgren]

@sdboyer Actually, I realized a few things after putting this together:

1. sort.Slice doesn't exist either on 1.7. So calling that is not actually a solution. The reason the tests are not failing is that they currently only run for >=1.9. If we want to implement support for older Go versions then we could copy the definition of func SliceStable(slice interface{}, less func(i, j int) bool) into sort_pre_go_1_8.go. Its implementation has remained the same since Go 1.8.

2. Like I wrote in Dep installation fails on Go version 1.7.x #1967, likeIy a better way to install Dep between Go versions (e.g. on Travis) is to just call curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh. So possibly that also eliminates the need to expand the backward compatibility for the source code.

[jmank88]

Is this actually the only thing that prevents dep from compiling on Go 1.7?

For reference, dropping 1.7: #714 and 1.8: #1620