Add support for refresh_db in brew

It appears that refresh_db became a feature that package managers are require to support as `KeyError: 'pkg.refresh_db'` was the error I was getting before this commit. I've not had a chance to fully test this fix, so I'd appreciate anyone else taking a look at this.

salt/modules/brew.py

@@ -124,7 +124,10 @@ def latest_version(*names, **kwargs):
         salt '*' pkg.latest_version <package name>
         salt '*' pkg.latest_version <package1> <package2> <package3>
     '''
-    kwargs.pop('refresh', True)
+    refresh = salt.utils.is_true(kwargs.pop('refresh', True))
+
+    if refresh:
+        refresh_db()
 
     if len(names) <= 1:
         return ''
@@ -179,6 +182,28 @@ def remove(name=None, pkgs=None, **kwargs):
     return salt.utils.compare_dicts(old, new)
 
 
+def refresh_db():
+    '''
+    Update the homebrew package repository.
+
+    CLI Example:
+
+    .. code-block:: bash
+
+        salt '*' pkg.refresh_db
+    '''
+    cmd = 'brew update'
+
+    homebrew_binary = __salt__['cmd.run']('brew --prefix') + "/bin/brew"
+    user = __salt__['file.get_user'](homebrew_binary)
+
+    if __salt__['cmd.retcode'](cmd, runas=user):
+        log.error('Failed to update')
+        return False
+
+    return True
+
+
 def install(name=None, pkgs=None, taps=None, options=None, **kwargs):
     '''
     Install the passed package(s) with ``brew install``