fix(brew_install): make subprocess.check_output return str

This fix makes the `brew_install` rule work on Python 3.

Signed-off-by: Pablo Santiago Blum de Aguiar <scorphus@gmail.com>

thefuck/rules/brew_install.py

@@ -6,7 +6,8 @@ from subprocess import check_output
 # Formulars are base on each local system's status
 brew_formulas = []
 try:
-    brew_path_prefix = check_output(['brew', '--prefix']).strip()
+    brew_path_prefix = check_output(['brew', '--prefix'],
+                                    universal_newlines=True).strip()
     brew_formula_path = brew_path_prefix + '/Library/Formula'
 
     for file_name in os.listdir(brew_formula_path):