Fix leftover reference to v4.0.0-alpha.6
Fix leftover reference to v4.0.0-alpha.6

Running `./build/change-version.js v4.0.0-alpha.6 v4.0.0` fixed this,
so the version change script works fine. I'm presuming instead this
change was just omitted from 35f80bb12e4e, and then wouldn't have
been caught by subsequent runs of `change-version`, since it only
ever replaces the exact old version string specified.

gijsbotje/bootstrap,Lyricalz/bootstrap,fschumann1211/bootstrap,inway/bootstrap,seanwu99/bootstrap,kvlsrg/bootstrap,joblocal/bootstrap,twbs/bootstrap,bardiharborow/bootstrap,joblocal/bootstrap,peterblazejewicz/bootstrap,kvlsrg/bootstrap,Hemphill/bootstrap-docs,m5o/bootstrap,inway/bootstrap,tjkohli/bootstrap,GerHobbelt/bootstrap,stanwmusic/bootstrap,tagliala/bootstrap,Hemphill/bootstrap-docs,tjkohli/bootstrap,yuyokk/bootstrap,bardiharborow/bootstrap,inway/bootstrap,Hemphill/bootstrap-docs,Hemphill/bootstrap-docs,nice-fungal/bootstrap,joblocal/bootstrap,fschumann1211/bootstrap,coliff/bootstrap,fschumann1211/bootstrap,joblocal/bootstrap,creativewebjp/bootstrap,Lyricalz/bootstrap,stanwmusic/bootstrap,gijsbotje/bootstrap,bootstrapbrasil/bootstrap,nice-fungal/bootstrap,gijsbotje/bootstrap,zalog/bootstrap,seanwu99/bootstrap,gijsbotje/bootstrap,creativewebjp/bootstrap,zalog/bootstrap,peterblazejewicz/bootstrap,creativewebjp/bootstrap,Lyricalz/bootstrap,coliff/bootstrap,stanwmusic/bootstrap,twbs/bootstrap,tagliala/bootstrap,bootstrapbrasil/bootstrap,bardiharborow/bootstrap,peterblazejewicz/bootstrap,m5o/bootstrap,yuyokk/bootstrap,nice-fungal/bootstrap,seanwu99/bootstrap,bootstrapbrasil/bootstrap,GerHobbelt/bootstrap,Lyricalz/bootstrap,bardiharborow/bootstrap,fschumann1211/bootstrap,tagliala/bootstrap,kvlsrg/bootstrap,yuyokk/bootstrap,yuyokk/bootstrap,GerHobbelt/bootstrap,stanwmusic/bootstrap,zalog/bootstrap,seanwu99/bootstrap,creativewebjp/bootstrap