Bump mozlog from 4.1 to 4.2.0 (#369)

Bumps [mozlog](https://wiki.mozilla.org/Auto-tools/Projects/Mozbase) from 4.1 to 4.2.0.

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

requirements/py27-tests.txt

@@ -1,5 +1,5 @@
 mock==3.0.5
-mozlog==4.1
+mozlog==4.2.0
 pyfxa==0.7.2
 pypom==2.2.0
 pytest==4.6.4