Python 3.4 requires PyYAML<=5.2

This works around a problem with the PyYAML distribution where PyPI
doesn't recognize the PyYAML 'python_requires' directive from setup.py.

.travis.yml

@@ -6,7 +6,7 @@ matrix:
   #- python: '2.7'
   #  env: PYRO="Pyro" JYTHON="org.python:jython-installer:2.7.0"
   - python: '3.4'
-    env: PYRO="Pyro4" YAML="pyyaml"
+    env: PYRO="Pyro4" YAML="pyyaml<=5.2"
   - python: '3.5'
     env: PYRO="Pyro4" YAML="pyyaml"
   - python: '3.6'
@@ -23,7 +23,7 @@ matrix:
     env: PYRO="Pyro4" YAML="pyyaml"
 install:
 - if [ -n "${JYTHON}" ]; then source install_jython.sh; fi
-- if [ -n "${YAML}" ]; then pip install pyyaml; fi
+- if [ -n "${YAML}" ]; then pip install "${YAML}"; fi
 - pip install xlrd
 - pip install $PYRO
 - pip install coverage