Fixed failing HTTP request when synthesizing TextToSpeech by removing…

… unneeded content-type header

- Fixed the failing HTTP request in `TextToSpeech().synthesize(_:voice:completionHandler)` by removing the unneeded JSON content-type header. Since no HTTP message is included in the request body, the content-type is superfluous.
  - Note that an extraneous content-type probably shouldn't cause a 400 response when the request body is empty for a GET request.

- Updated AlamofireObjectMapper version for Carthage compatibility and to fix [Carthage build error due to deployment targets](tristanhimmelman/AlamofireObjectMapper#63) without introducing any API changes.
  - Uses `~>` for maximum compatibility in the dependency graph

Cartfile

@@ -3,4 +3,4 @@ github "DaveWoodCom/XCGLogger" == 3.1.1
 github "Hearst-DD/ObjectMapper" == 1.0.1
 github "rhodgkins/SwiftHTTPStatusCodes" == 2.0.1
 github "daltoniam/Starscream" == 1.0.2
-github "tristanhimmelman/AlamofireObjectMapper" == 2.1.0
+github "tristanhimmelman/AlamofireObjectMapper" ~> 2.1.1

Cartfile.resolved

@@ -3,4 +3,4 @@ github "Hearst-DD/ObjectMapper" "1.0.1"
 github "daltoniam/Starscream" "1.0.2"
 github "rhodgkins/SwiftHTTPStatusCodes" "2.0.1"
 github "DaveWoodCom/XCGLogger" "Version_3.1.1"
-github "tristanhimmelman/AlamofireObjectMapper" "2.1.0"
+github "tristanhimmelman/AlamofireObjectMapper" "2.1.3"

WatsonDeveloperCloud/TextToSpeech/TextToSpeech.swift

@@ -87,7 +87,6 @@ public class TextToSpeech: WatsonService {
             endpoint: Constants.synthesize,
             authStrategy: authStrategy,
             accept: .OPUS,
-            contentType: .JSON,
             urlParams: urlParams)
 
         // execute request