Implement fallback print method for testing environment in case mruby-print GEM wasn't included.
Move declaration of kernel to the beginning of the method
[doc] Add minimal build configuration

In relation to #1225.

Adds a minimal build configuration for libmruby which contains as less as possible.
Merge pull request #1239 from bovi/fallback-print

[TestEnv] Fallback print method
Merge pull request #1240 from bovi/patch-1

[doc] Add minimal build configuration