tabline: do not confuse Vim
tabline: do not confuse Vim

looks like this:
```
let a=(condition ? s:var:'')
```
confuses older Vims and it complains about a missing colon. So make
parsing a bit easier and add a space in front of the second colon.

fixes #1629

yzhou61/vim-airline,bling/vim-airline,tschundler/dotfiles,chrisbra/vim-airline,tschundler/dotfiles,tschundler/dotfiles,unphased/vim-airline,rchicoli/vim-airline