Support multiple Lagom versions

[ignasi35]

It'd be good, as Lagom evolves to be able to generate the code of different Lagom version if/when APIs in Lagom break or include new features.

E.g. the current status of the code generator was tested with Lagom 1.4.0-SNAPSHOT. The Java code generator would work with Lagom 1.1.0+ but not the scala code generator (the Lagom scala API was introduced in Lagom 1.3.x). Same goes if generating code for an old version that didn't support message publishing.