change 'lastrevid' to 'baserevid' in API calls

Currently, Site.changeClaimTarget is throwing an API warning: "Unrecognized parameter: 'lastrevid'"; just changing four characters solve the issue.
Also changed in editSource, as it already is in Site.removeClaims.

Change-Id: Iaf8a1405694cb7367916a72f54e0b53ededdfe27