Fix AccountMailerTest failures [Finishes #154110059]
Fix AccountMailerTest failures [Finishes #154110059]

AccountMailerTest::fix_mac_vs_pc was replacing `<br />\n` in generated html mail with `<br/>`.
Tests failed because they were expecting `<br />\n`.
(This was not a problem before updating gems, because the generated html mail then had `<br />\r\n` rather than `<br />\n`.)

For more detailed discussion see https://www.pivotaltracker.com/story/show/154110059.

pellaea/mushroom-observer,pellaea/mushroom-observer,JoeCohen/mushroom-observer,JoeCohen/mushroom-observer,MushroomObserver/mushroom-observer,pellaea/mushroom-observer,pellaea/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,JoeCohen/mushroom-observer,MushroomObserver/mushroom-observer,JoeCohen/mushroom-observer,MushroomObserver/mushroom-observer,pellaea/mushroom-observer,JoeCohen/mushroom-observer,JoeCohen/mushroom-observer,JoeCohen/mushroom-observer,MushroomObserver/mushroom-observer,pellaea/mushroom-observer,pellaea/mushroom-observer