Update requirements.txt for libmozdata (#509)

requirements.txt

@@ -4,7 +4,7 @@ certifi>=2017.4.17
 chardet<3.1.0,>=3.0.2
 httplib2>=0.9.1
 humanize>=0.5.1
-libmozdata>=0.1.49
+libmozdata>=0.1.52
 path.py==2.2.2
 pep8==0.6.1
 pyflakes==0.5.0