[Keyboard] Changed the font weight from "faux" bold to Medium

[patrykdesign]

Bold doesn't exist, so the system applies an ugly stroke.
So I changed Bold / 600 to Medium / 500.

[patrykdesign]

@timdream can you have a look, that this change it didn't break anything. Thanks!

[timdream]

Have you verify the change on the phone? @alivedise shown me that from his tests he is not sure 500 is indeed medium.

This patch doesn't break anything but we need to make sure it actualy works.

[vingtetun]

@patrykdesign Is it still needed if https://bugzilla.mozilla.org/show_bug.cgi?id=804109 lands?

[patrykdesign]

@vingtetun technically yes, but https://bugzilla.mozilla.org/show_bug.cgi?id=804109 will also fix this visual problem and many others.

[patrykdesign]

Here is how the fix looks on device http://cl.ly/image/3K2Z023u3m0A