add ability to specify the maximum TLS version

Signed-off-by: Lyas Spiehler <lspiehler@gmail.com>

config/http_config.go

@@ -21,6 +21,7 @@ import (
 	"crypto/x509"
 	"encoding/json"
 	"fmt"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"net/url"
@@ -826,6 +827,8 @@ type TLSConfig struct {
 	InsecureSkipVerify bool `yaml:"insecure_skip_verify" json:"insecure_skip_verify"`
 	// Minimum TLS version.
 	MinVersion TLSVersion `yaml:"min_version,omitempty" json:"min_version,omitempty"`
+	// Maximum TLS version.
+	MaxVersion TLSVersion `yaml:"max_version,omitempty" json:"max_version,omitempty"`
 }
 
 // SetDirectory joins any relative file paths with dir.