Avoid forcing infos of some symbols in InfoTransforms

Provides the "mayChange" hook to exclude symbols from being completed
prior to a transformInfo.
Setting travis to use oracle jdk8
Avoid computing info of TypeRefs for Object <-> Any comversions in ClassfileParser

Another bit to help avoid CyclicReferences when reading Java classes
Merge pull request #223 from dotty-staging/fix/elimRepeatedTransformSym

Avoid forcing infos of some symbols in InfoTransforms