make CI build faster

[PeterDaveHello]

No description provided.

[coveralls]

Coverage remained the same when pulling 33b1f5b on PeterDaveHello:patch-1 into d21f532 on angular-ui:master.

[thgreasi]

With the current size of the repo, I didn't see any gains comparing this PR's build time and for example the svg PR??

[PeterDaveHello]

Because the deviation range is very big, but it can still help, just like we don't need the whole repo to CI build, do use git shallow clone, the default value of depth is 50 on travis CI, but usually we don't need so many revisions to build, a lower number can help to save the bandwidth and time.

[thgreasi]

The merging of v0.13.x-dev branch into master contained more than 20 commits, would that be a problem with this PR?
Also, while searching Github I found that some repos have reverted their explicit git setting.

[PeterDaveHello]

I think that won't be a problem.

[thgreasi]

I will be honest... I'm afraid enough to not merge this.
I appreciate your commit and plan to rebase it after each major release of ui-sortable. At the point that the CI build gets faster, I will reopen and merge it.

[PeterDaveHello]

No problem.