It's just a guess, maybe we are lucky.
The original problem is that travis checks out tags without branches,
and thus checking out master does only work if travis runs on master.

With tags, it will only heckout and locally know the tag in question.
The changes should allow it to retry and create the master branch
instead.

In response to
https://github.com/gitpython-developers/GitPython/pull/408/files/5de21c7fa2bdd5cd50c4f62ba848af54589167d0..aae2a7328a4d28077a4b4182b4f36f19c953765b#r59722704

Just to get the tests green again for py2.7.
Deleting the test isn't the right way to do it, but the one of a
maintainer who wants to spend no more time than possibly necessary.

Previously travis would fail to clone the special testing repository
as SSH access requires a deploy key to be set. The latter has been
configured and is now added in clear-text.

This is save as it is only good for read-only access to a test
repository, mis-use cannot happen that way.

Related to #301

As it seems to contain custom inforamtion that we want to keep.
Now we are appending to it ...

* config parser now handles quoted values correctly. This doesn't hamper
  multi-line support.
* added regression test to travis to assure we will be warned if we
  rewrite and break the user's .gitconfig file
* only rewrite configuration files if we actually called a mutating
  method on the writer. Previously it would always rewrite it.

Fixes #285