Keep support for Python 3.4 - pin PyYAML properly (#348)

Reverts 1937372, continuation of #346.

Author: blueyed

.travis.yml

@@ -25,6 +25,9 @@ jobs:
     - name: Python 3.5
       python: "3.5"
       env: TOXENV=py35-cov
+    - name: Python 3.4
+      python: "3.4"
+      env: TOXENV=py34-cov
 
 install:
   - pip install tox

setup.py

@@ -3,7 +3,8 @@
 
 
 install_requires = [
-    'PyYAML>=3.11',
+    'PyYAML<=5.2;python_version<"3.5"',
+    'PyYAML>=3.11;python_version>="3.5"',
     'ansicolor>=0.2.4',
     'chardet>=2.3.0',
     'setuptools>=36.2.2',  # for enhanced marker support (used below).
@@ -59,6 +60,7 @@ def read_readme():
         'Topic :: Text Editors',
         'Programming Language :: Python',
         'Programming Language :: Python :: 2.7',
+        'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',