Fix macro when interpolated argument is Throwable

[dwickern]

When passing an interpolated string with a Throwable, the Scala 2 macro incorrectly calls the non-format overload:

public void error(String msg, Throwable t);

Coerce all arguments to AnyRef so that the format overload is used:

public void error(String format, Object arg);

Fixes #216
Fixes #95

[kamilkloch]

When can we expect a release with a fix? :)

[mox692]

The release that includes this change has been published as v3.9.5. However, unfortunately, it seems that the original problem described in the issue has not been resolved even in the latest release :(