Bump mozlog from 3.10 to 4.0 (#326)

Bumps [mozlog](https://wiki.mozilla.org/Auto-tools/Projects/Mozbase) from 3.10 to 4.0.

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements/py27-tests.txt

@@ -1,5 +1,5 @@
 mock==2.0.0
-mozlog==3.10
+mozlog==4.0
 pyfxa==0.6.0
 pypom==2.2.0
 pytest==4.2.0