chore(travis): custom merge pubspec.lock for g3v1x rebases

pubspec.lock changes will never really apply cleanly to g3v1x.  What we
wnat to capture, is whether Travis tests will pass if we apply the
pubspec.yaml changes to the g3v1x branch and then regenerate the
pubspec.lock file.  This commit inserts a custom merge driver for git to
ignore changes to the pubspec.lock file during merging to achieve this.