installer assumes layout in erb

[rubydesign]

./bin/rails g opal:install
raises

No such file or directory @ rb_sysopen - app/views/layouts/application.html.erb 

if the layout is not erb.

Since it mentions the haml filter below, i thought i'll mention this. As a long time haml users i have not used erb for ages.
Can give this a shot if you are too busy and accepting pr

PS: I'm just trying opal out again after a long time and wanted to see the haml integration.
Haml btw has changed filter handling and the code is not compatible with haml 6 , maybe update the gemspec haml < 6.0
I just did the upgrade for ruby2js, so maybe can give that a show too, if my tryouts go well.