Config: Detect user config using os.path.expanduser()

Instead of `$HOME`, since the former works when `$HOME` is not set.

[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=828033#10

docs/configuration.rst

@@ -16,7 +16,7 @@ following locations (by order of preference):
 
 - ``.yamllint`` in the current working directory
 - ``$XDG_CONFIG_HOME/yamllint/config``
-- ``$HOME/.config/yamllint/config``
+- ``~/.config/yamllint/config``
 
 Finally if no config file is found, the default configuration is applied.
 

yamllint/cli.py

@@ -87,13 +87,11 @@ def run(argv=None):
         sys.exit(-1)
 
     # User-global config is supposed to be in ~/.config/yamllint/config
-    user_global_config = None
     if 'XDG_CONFIG_HOME' in os.environ:
         user_global_config = os.path.join(
             os.environ['XDG_CONFIG_HOME'], 'yamllint', 'config')
-    elif 'HOME' in os.environ:
-        user_global_config = os.path.join(
-            os.environ['HOME'], '.config', 'yamllint', 'config')
+    else:
+        user_global_config = os.path.expanduser('~/.config/yamllint/config')
 
     try:
         if args.config_data is not None:
@@ -104,7 +102,7 @@ def run(argv=None):
             conf = YamlLintConfig(file=args.config_file)
         elif os.path.isfile('.yamllint'):
             conf = YamlLintConfig(file='.yamllint')
-        elif user_global_config and os.path.isfile(user_global_config):
+        elif os.path.isfile(user_global_config):
             conf = YamlLintConfig(file=user_global_config)
         else:
             conf = YamlLintConfig('extends: default')