.travis.yml: use the Limnoria's testing branch.

* This might help if something changes in testing and causes breakage here too.
* testing branch is updated more often than pip which was forgotten often in the past and is based on master branch anyway.
Merge pull request #206 from Mkaysi/patch-1

.travis.yml: use the Limnoria's testing branch.
.travis.yml: install Limnoria's requirements too.
.travis.yml: install Limnoria's requirements

before Limnoria itself.
Merge pull request #207 from Mkaysi/patch-1

.travis.yml: install Limnoria's requirements too.
README.md & .travis.yml: add `--upgrade` to pip

I have explained this in other repositories, please see them.