Set git clone depth to 10 for Travis to make it faster

[BBosman]

Just noticed that on the Apache Cordova project they've set the git clone depth for Travis to 10 as a performance optimization.

Could be an interesting optimization for Bootstrap as well.

Related Cordova commit

[XhmikosR]

This will bite us in the ass if a PR is frequently rebased. But I guess time will tell.

[hnrch02]

@XhmikosR Unless someone rebases with --committer-date-is-author-date that's not gonna happen.

[XhmikosR]

I have seen this to fail numerous times. Say you cherry pick a patch and make a new PR.

Anyway, time will tell how frequently will this affect us.

[hnrch02]

When I cherry-pick the committer date gets set to right now.

[cvrebert]

@XhmikosR What problem does this cause for rebases exactly? Genuinely curious.
AFAIK, there's nothing in our Travis build that needs the git history beyond the head commit.

[XhmikosR]

@cvrebert: the issue can happen when force push is used in a branch and the commits exceed the depth before Travis manages to start a build; then it will error out with a commit doesn't exist message.

10 is a nice depth, but I would probably go with a little higher.. Usually our PRs aren't so big, so we probably won't hit the issue at all with a bigger depth.