Change py.test to pytest (#338)

Author: Dovacody

CONTRIBUTING.md

@@ -322,7 +322,7 @@ version) of your plot.
 Run the following from the repository root:
 
 ```bash
-py.test --mpl-generate-path=baseline pygmt/tests/NAME_OF_TEST_FILE.py
+pytest --mpl-generate-path=baseline pygmt/tests/NAME_OF_TEST_FILE.py
 ```
 
 This will create a `baseline` folder with all the plots generated in your test

pygmt/__init__.py

@@ -49,7 +49,7 @@ def test(doctest=True, verbose=True, coverage=False, figures=True):
     """
     Run the test suite.
 
-    Uses `py.test <http://pytest.org/>`__ to discover and run the tests. If you
+    Uses `pytest <http://pytest.org/>`__ to discover and run the tests. If you
     haven't already, you can install it with `conda
     <http://conda.pydata.org/>`__ or `pip <https://pip.pypa.io/en/stable/>`__.