JRuby support

[katafrakt]

This is a tracking issue for initiative of making Hanami CLI compatible with JRuby. This is probably the largest step in making the whole framework compatible.

To my knowledge, this is what needs to be done:

• Build assets without using fork (Use threads instead of forking to compile slices assets #404)
• SQlite3 JDBC compatibility (Support SQLite3 JDBC #434)
• MySQL JDBC compatibility
• PostgreSQL JDBC compatibility
• Make sure hanami new generates correct Gemfile and .env in JRuby