Add "Priority Speaker" permission as constant (#628)

* Bump to v0.17.0

* Add members from GuildMembersChunk to state (#454)

* Revert "Add members from GuildMembersChunk to state (#454)" (#455)

This reverts commit e4487b30d4d846b1fdc08fd3982bd5b9965a8cc9.

* added clarification when initializing discordgo

from the code I ran on my system with the latest changes this seems to be the syntax for the authentication tokens. I'm guessing it was just never updated.
Let me know if this is incorrect. 
Thanks!

* travis: update go versions

* Add "Priority Speaker" permission constant

The permission "Priority Speaker" was missing as constant. Permission has been added as constant and to the "PermissionAllVoice" constant as well.

README.md

@@ -73,7 +73,7 @@ Construct a new Discord client which can be used to access the variety of
 Discord API functions and to set callback functions for Discord events.
 
 ```go
-discord, err := discordgo.New("authentication token")
+discord, err := discordgo.New("Bot " + "authentication token")
 ```
 
 See Documentation and Examples below for more detailed information.

structs.go

@@ -872,6 +872,7 @@ const (
 	PermissionVoiceDeafenMembers
 	PermissionVoiceMoveMembers
 	PermissionVoiceUseVAD
+	PermissionVoicePrioritySpeaker = 1 << (iota + 2)
 )
 
 // Constants for general management.
@@ -907,7 +908,8 @@ const (
 		PermissionVoiceMuteMembers |
 		PermissionVoiceDeafenMembers |
 		PermissionVoiceMoveMembers |
-		PermissionVoiceUseVAD
+		PermissionVoiceUseVAD |
+		PermissionVoicePrioritySpeaker
 	PermissionAllChannel = PermissionAllText |
 		PermissionAllVoice |
 		PermissionCreateInstantInvite |