For now, do not have travis build tip.

The location of go vet moved but the new version only works with tip
while the old one works with release.  So for now, not building tip.
Add invalidateblock and reconsiderblock.
Update TracisCI for Go 1.4 release.

Our policy is to only ensure the code compiles for the latest Go release
minus one version.  Since Go 1.4 has now been released, this commit
updates TravisCI to remove Go 1.2 from the build matrix and add Go 1.3.

Also, go vet changed locations in between Go 1.3 and Go 1.4, so a check
has been added to download the appropriate version based on which version
of Go is running the integration tests.

Finally, the previous removed Go tip entry in the build matrix has been
reintroduced since it will now work again.
Correct recent TravisCI go vet fetch script.
Be more specific in TravisCI go vet version test.
Accept nil string for account name