Don't fail on class file version downgrade

[Egorand]

When the code that was previously compiled against Java 7 is being compiled against Java 6, Japicmp reports class file version incompatibility:

Comparing binary compatibility of  against 
***! MODIFIED CLASS: PUBLIC FINAL okio.Options  (not serializable)
	***! CLASS FILE FORMAT VERSION: 50.0 <- 51.0

Since this doesn't really break compatibility for the callers of this code (Java 7 and higher can compile against binaries built for Java 6), I suggest not failing the build when this incompatibility is encountered, and potentially reporting it as a warning.