[vim] Improve binary detection
[vim] Improve binary detection

/cc @alerque

- Ask for user confirmation before running `install --bin`
- Removed `s:fzf_rb` since `install --bin` will create a wrapper
  executable that just runs Ruby version on the platforms where prebuilt
  binaries are not available.

NgaNguyenDuy/fzf,jebaum/fzf,qiemem/fzf,janlazo/fzf,hyiltiz/fzf,hyiltiz/fzf,junegunn/fzf,junegunn/fzf,me-and/fzf,pokey/fzf,tomfitzhenry/pkg-fzf,bobpattersonjr/fzf,junegunn/fzf,infokiller/fzf,bobpattersonjr/fzf,wilywampa/fzf,d4ndo/fzf,alerque/fzf,tomfitzhenry/pkg-fzf,alerque/fzf,pokey/fzf,brosander/fzf,hyiltiz/fzf,mjwestcott/fzf,brosander/fzf,wilywampa/fzf,noscript/fzf,me-and/fzf,infokiller/fzf,wilywampa/fzf,pokey/fzf,infokiller/fzf,lululau/fzf,lululau/fzf,hudak/fzf,hudak/fzf,bobpattersonjr/fzf,fazibear/fzf,janusnic/fzf,mjwestcott/fzf,alerque/fzf,lululau/fzf,janlazo/fzf,d4ndo/fzf,gregory-nisbet/fzf,noscript/fzf,qiemem/fzf,mhartington/fzf,gregory-nisbet/fzf,jebaum/fzf,brosander/fzf,mhartington/fzf,janusnic/fzf,hudak/fzf,tomfitzhenry/pkg-fzf,janusnic/fzf,fazibear/fzf,NgaNguyenDuy/fzf,mhartington/fzf,NgaNguyenDuy/fzf,d4ndo/fzf,janlazo/fzf,fazibear/fzf,jebaum/fzf,stevenqzhang/fzf,mjwestcott/fzf,stuha/wbr-zsh,me-and/fzf,qiemem/fzf,stevenqzhang/fzf,stevenqzhang/fzf,noscript/fzf,gregory-nisbet/fzf