Since the latest golint no longer works with on Go 1.4 and Go 1.4 is no
longer officially supported by btcsuite, remove it from the
configurations tested by TravisCI.
Now that Go 1.6 has been released, update the required Go version in the
README to 1.5 and add Go 1.6 to the configurations tested by TravisCI.
Also, while here, update the Go 1.4 and 1.5 versions tested by TravisCI
to the latest point releases.
This commit causes TravisCI to run several tools on each pull request and
commit to help ensure the code quality remains high. This includes gofmt,
goimports, golint, go vet, the race detector, and coverage stats.
Also, it instructs TravisCI to use nicer container-based builds.
