doc(README.md): add coverity scan badge

[skip ci]
doc(README.md): use .svg Lean logo; centering

[skip ci]
doc(README.md): Use img.shields.io/ icon for CDash

[skip ci]
chore(travis): fix bitbucket push

Problem
=======

For efficiency, travis-ci uses shallow clone by doing

    git clone --depth=50 ...

which checks out recent 50 commits from the repo.

However, recent changes on bitbucket side rejects a shallow clone of a
repository.

Solution
========

We do

    git fetch --unshallow;

before we push to other repositories.
chore(travis): fix bitbucket push