[MRG] move from unittest to pytest (#90)

* use pytest rather than unittest

* update README.txt

Author: wdevazelhes

.travis.yml

@@ -8,4 +8,4 @@ before_install:
   - pip install --upgrade pip
   - pip install wheel
   - pip install numpy scipy scikit-learn
-script: python setup.py test
+script: pytest test

README.rst

@@ -29,7 +29,8 @@ Run ``pip install metric-learn`` to download and install from PyPI.
 
 Run ``python setup.py install`` for default installation.
 
-Run ``python setup.py test`` to run all tests.
+Run ``pytest test`` to run all tests (you will need to have the ``pytest``
+package installed).
 
 **Usage**