Making conversions work with type-class materializers.

Fixes #33 #19

Review by @gzm0 @hubertp
Merge pull request #46 from vjovanov/clean-conversions

Making conversions work with type-class materializers.
Disable 2.12.0-SNAPSHOT build on travis
Merge pull request #49 from hubertp/topic/disable-2.12-travis

Disable 2.12.0-SNAPSHOT build on travis
R is now renamed to Rec.

Review by @gzm0