update Travis config to reflect that PyYAML 5.x is no longer compatib…

…le with Python 2.6

.travis.yml

@@ -55,8 +55,10 @@ before_install:
     - if [ "x$TRAVIS_PYTHON_VERSION" == 'x2.6' ]; then pip install 'python-daemon<2.0'; fi
     # autopep8 1.3.4 is last one to support Python 2.6
     - if [ "x$TRAVIS_PYTHON_VERSION" == 'x2.6' ]; then pip install 'autopep8<1.3.5'; else pip install autopep8; fi
+    # PyYAML 5.x no longer supports Python 2.6
+    - if [ "x$TRAVIS_PYTHON_VERSION" == 'x2.6' ]; then pip install 'PyYAML<5.0'; else pip install PyYAML; fi
     # optional Python packages for EasyBuild
-    - pip install GC3Pie pycodestyle python-graph-dot python-hglib PyYAML requests
+    - pip install GC3Pie pycodestyle python-graph-dot python-hglib requests
     # git config is required to make actual git commits (cfr. tests for GitRepository)
     - git config --global user.name "Travis CI"
     - git config --global user.email "travis@travis-ci.org"