build: Remove git-depth:3 from Travis CI config

build: Remove git-depth:3 from Travis CI config

Follows-up 236b7ae849.

The git-clone protocol does not support checking out a specific
commit directly. It requires that you clone first from a specific
branch (with an optional specified limited depth) and then you
can checkout a commit.

Limiting the depth means that the Travis CI build fails if when
the build starts at a time where there are at least N commits
in master after the one being tested.

This can happen if more than 3 commits merge within 15 minutes
of time (which is how long our Travis build takes roughly).
While that might seem unlikely given our Jenkins build take
about the same amount of time, our Zuul is able to anticipate
ahead of a merge and actually test many commits at once and then
once they all pass, merge them all at once.

Aside from that, one can also click "Restart" as repo admin in
Travis, which always fails for a past build basically.

Change-Id: I2a46821d6d78e637f03de0c4aa25a3488253ff1a