Fixed Travis URL in CONTRIBUTING and still trying to deal with pytest

error in Travis tests.

Author: rbeyer

.travis.yml

@@ -9,7 +9,9 @@ python:
   - "pypy3"
 
 # command to install dependencies
-install: pip install -r requirements.txt
+install: 
+  - pip install -r requirements.txt
+  - pip install --upgrade pytest
 
 # lint before running tests
 before_script: flake8 pvl tests

CONTRIBUTING.rst

@@ -101,7 +101,7 @@ Before you submit a pull request, check that it meets these guidelines:
    feature to the list in HISTORY.rst and potentially update the README.rst 
    or other documentation files.
 3. The pull request should work for Python 3.6, 3.7, and 3.8. Check
-   https://travis-ci.org/planetarypy/pvl/pull_requests
+   https://travis-ci.org/github/planetarypy/pvl
    and make sure that the tests pass for all supported Python versions.
 
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