TST: CI maintenance (#246)

This PR updates 

* Travis CI's installation of the testing dependencies to use conda rather than pip, avoiding an issue where Python 3.6 failed to install the certifi package
* Appveyor's `pytest` call to correctly return the error code from the `pytest.main` function. Previously, the Python script exited at the end of the command with a successful error code, regardless of the status of the test suite.

The behavior was verified by writing a dummy `assert 1 == 2` test that caused all tests to fail (ade9aca), then removing that test to observe all tests succeeded (4a80675).

Closes #238 

* Throwaway: Known CI failure

* Travis: Use conda to install test dependencies

* Add --yes to conda install

* Appveyor: Exit with exit code returned by pytest.main function

* Revert "Throwaway: Known CI failure"

CI tests should go from red to green now

This reverts commit 476c4bb.

.travis.yml

@@ -40,14 +40,14 @@ install:
       conda activate condaenv
       echo "!!! Installing pycalphad dependencies via conda"
       conda install --yes python=$TRAVIS_PYTHON_VERSION numpy scipy matplotlib pytest pytest-cov pandas sympy pyparsing dask dill python-symengine xarray cython cyipopt
+      echo "!!! conda installing test packages"
+      conda install --yes sphinx sphinx_rtd_theme coveralls ipython
       echo "!!! pip pycalphad as editable"
       pip install -e .
 
 # Run test
 script:
   - |
-      echo "!!! pip installing test packages"
-      pip install sphinx sphinx_rtd_theme coveralls ipython
       echo "!!! conda list output"
       conda list
       echo "!!! matplotlib py27 fix"

appveyor.yml

@@ -53,4 +53,4 @@ install:
 build: false
 
 test_script:
-  - "python -c \"import pytest ; pytest.main()\" -s -v pycalphad"
+  - "python -c \"import pytest ; exit(pytest.main())\" -s -v pycalphad"