decoder: put the most-likely case first to help branch predictions
decoder: moves some operations out of cp's inner loop
decoder: comments++
travis: update for go1.1

We have some code that fails to build with go1.0, so update the travis
config.  Note that we'll need to bump it again in a few weeks when 1.2
is released.
Merge pull request #6 from dgryski/decode-optimize

Some small tweaks to the decoder.
Merge pull request #7 from dgryski/travis-go11

Update travis for Go 1.1