Require 'erubis' if needed in erb filter

Requiring erubis/tiny can cause the erb fiter to fail if erubis hasn't
been explicitly required elsewhere.

See https://github.com/rtomayko/tilt/pull/183

As a workaround require 'erubis' if needed when compiling the erb
filter.

Strictly speaking I think this more of a Tilt issue, but this is a workaround until we can depend on a version of Tilt with the fix in.