Fix deprecation warning on Rails 6

[r7kamura]

When trying rails 6.0.0.beta3 with hamlit 2.9.2, I found a following deprecation warning:

DEPRECATION WARNING: Single arity template handlers are deprecated.  Template handlers must
now accept two parameters, the view object and the source for the view object.
Change:
  >> Hamlit::RailsTemplate#call(template)
To:
  >> Hamlit::RailsTemplate#call(template, source)

This pull request brings hamlit template handler up to work with Rails 6 and no deprecation warnings.

See also:

• https://github.com/rails/rails/pull/35119
• https://github.com/haml/haml/pull/1008
• https://github.com/rails/coffee-rails/pull/101
• https://github.com/rails/jbuilder/pull/453