setup.py: List 'seaborn' as 'test' dependency

Used in Parameter\ Heatmap.ipynb

.travis.yml

@@ -39,7 +39,7 @@ before_install:
   - set -e
 
 install:
-  - pip install .
+  - pip install .[test]
 
 script:
   - BOKEH_BROWSER=none time catchsegv python setup.py test

setup.py

@@ -40,6 +40,9 @@ if __name__ == '__main__':
                 'jupytext >= 0.7.0',
                 'nbconvert',
             ],
+            'test': [
+                'seaborn',
+            ]
         },
         test_suite="setup._discover_tests",
         python_requires='>=3.4',