TST: update dependencies in .travis.yml

- install pytest using pip
- install emcee/numdifftools using pip and only when version == latest
- use pip because conda does not have the latest versions for all Python releases

.travis.yml

@@ -26,15 +26,16 @@ before_install:
     - conda info -a
 
 install:
-    - if [[ $version == minimum && $TRAVIS_PYTHON_VERSION != 3.6 && $TRAVIS_PYTHON_VERSION != 3.7-dev ]]; then conda create -q -n test_env python=$TRAVIS_PYTHON_VERSION numpy=1.10 scipy=0.17 six=1.10 pytest; fi
-    - if [[ $version == minimum && $TRAVIS_PYTHON_VERSION == 3.6 ]]; then conda create -q -n test_env python=$TRAVIS_PYTHON_VERSION numpy=1.11.2 scipy=0.18 six=1.10 pytest; fi
-    - if [[ $version == minimum && $TRAVIS_PYTHON_VERSION == 3.7-dev ]]; then conda create -q -n test_env python=3.7 numpy=1.11.3 scipy=1.1 six=1.11 pytest; fi
-    - if [[ $version == latest && $TRAVIS_PYTHON_VERSION != 3.7-dev ]]; then conda create -q -n test_env python=$TRAVIS_PYTHON_VERSION numpy scipy six pandas matplotlib dill pytest; fi
-    - if [[ $version == latest && $TRAVIS_PYTHON_VERSION == 3.7-dev ]]; then conda create -q -n test_env python=3.7 numpy scipy six pandas matplotlib dill pytest; fi
+    - if [[ $version == minimum && $TRAVIS_PYTHON_VERSION != 3.6 && $TRAVIS_PYTHON_VERSION != 3.7-dev ]]; then conda create -q -n test_env python=$TRAVIS_PYTHON_VERSION numpy=1.10 scipy=0.17 six=1.10; fi
+    - if [[ $version == minimum && $TRAVIS_PYTHON_VERSION == 3.6 ]]; then conda create -q -n test_env python=$TRAVIS_PYTHON_VERSION numpy=1.11.2 scipy=0.18 six=1.10; fi
+    - if [[ $version == minimum && $TRAVIS_PYTHON_VERSION == 3.7-dev ]]; then conda create -q -n test_env python=3.7 numpy=1.11.3 scipy=1.1 six=1.11; fi
+    - if [[ $version == latest && $TRAVIS_PYTHON_VERSION != 3.7-dev ]]; then conda create -q -n test_env python=$TRAVIS_PYTHON_VERSION numpy scipy six pandas matplotlib dill; fi
+    - if [[ $version == latest && $TRAVIS_PYTHON_VERSION == 3.7-dev ]]; then conda create -q -n test_env python=3.7 numpy scipy six pandas matplotlib dill; fi
     - source activate test_env
+    - pip install pytest
     - pip install asteval
-    - pip install emcee
     - pip install uncertainties
+    - if [[ $version == latest ]]; then pip install emcee numdifftools; fi
     - python setup.py install
     - conda list