Add -f flag to go get in .travis.yml

Under go 1.4 when using -u the -f flag must be provided to allow cloning
a package from git:// urls. See [1]

1: https://github.com/golang/go/commit/ea295a4cfbb0641f58c41d3722e7e8fb3b3f493f

Signed-off-by: Kris Hicks <krishicks@gmail.com>