Don't report coverage to coveralls.io on Appveyor for PRs

For coverage reporting on Appveyor it's necessary to set
COVERALLS_REPO_TOKEN env variable. For security reasons it's
not available when building pull requests, so don't call 'coveralls'
in that case. To compensate, always run 'coverage report'.

appveyor.yml

@@ -18,4 +18,5 @@ test_script:
   - pyflakes .
   - pep8 .
   - nosetests
-  - coveralls
+  - coverage report
+  - if not "%COVERALLS_REPO_TOKEN%"=="" coveralls