Add dependency on Atom v0.121.0
Add dependency on Atom v0.121.0

We're now using APIs that aren't present in v0.120.0, so only let people
upgrade if they have a recent version of Atom itself.

crazymykl/git-blame,alexcorre/git-blame,aotarola/git-blame,MichaelAquilina/git-blame,SlimeQ/git-graph