Go 1.10 support (#223)

* Update Travis CI to use latest Go releases

* Fix go vet issues for Go 1.10

Starting in Go 1.10, the `go test` command now automatically runs `go
vet`. This commit fixes two issues flagged by vet that cause `go test`
to fail.

* Fix gofmt issue for Go 1.10

Go 1.10 introduced a small formatting change with comments in empty
multiline slice definitions.  This commit attempts to move the offending
comment in such a way that all version of gofmt will agree on its
location.

* Remove go-vet from test.sh

Starting in Go 1.10, the `go test` command automatically runs `go vet`,
so we don't need to run `go vet` explicitly from within test.sh.

Fixes #222

cmd/tomljson/main.go

@@ -17,13 +17,12 @@ import (
 
 func main() {
 	flag.Usage = func() {
-		fmt.Fprintln(os.Stderr, `tomljson can be used in two ways:
-Writing to STDIN and reading from STDOUT:
-  cat file.toml | tomljson > file.json
-
-Reading from a file name:
-  tomljson file.toml
-`)
+		fmt.Fprintln(os.Stderr, "tomljson can be used in two ways:")
+		fmt.Fprintln(os.Stderr, "Writing to STDIN and reading from STDOUT:")
+		fmt.Fprintln(os.Stderr, "  cat file.toml | tomljson > file.json")
+		fmt.Fprintln(os.Stderr, "")
+		fmt.Fprintln(os.Stderr, "Reading from a file name:")
+		fmt.Fprintln(os.Stderr, "  tomljson file.toml")
 	}
 	flag.Parse()
 	os.Exit(processMain(flag.Args(), os.Stdin, os.Stdout, os.Stderr))