Update .travis.yml to dynamically load go tools
Update .travis.yml to dynamically load go tools

* Grabs correct version of `go vet` and `go cover` based on `TRAVIS_GO_VERSION`.
* Adds Go 1.4 to build matrix.
cloudfoundry/dropsonde,hpcloud/dropsonde,hpcloud/dropsonde,cloudfoundry/dropsonde