Travis CI Not building Unity3D Project for Windows (on OS X Server)

My project builds on Travis CI using the following build.sh:

 #! /bin/sh
 
 project="Cygnus"
 
 echo "Attempting to build $project for Windows"
 /Applications/Unity/Unity.app/Contents/MacOS/Unity \
   -batchmode \
   -nographics \
   -silent-crashes \
   -logFile $(pwd)/unity.log \
   -projectPath $(pwd) \
   -executeMethod Build.BuildGame \
   -quit
 
 echo '\n\nLogs from build\n'
 cat $(pwd)/unity.log

and it executes Build.BuildGame:

 static void BuildGame() {
     BuildPipeline.BuildPlayer(new string[] { "Assets/Scenes/DebugScene.unity" }, "Build/Windows/Cygnus.exe", BuildTarget.StandaloneWindows, BuildOptions.Development);
     File.Copy("version", "Build/Windows/version");
 }

When i run this on my local machine (Windows 10 64 bit), it builds perfectly, with the Compiler Arguments being:

 -----Compiler Commandline Arguments:
 Filename: "C:\Program Files\Unity\Editor\Data\Mono\bin\mono.exe"
 Arguments: "C:\Program Files\Unity\Editor\Data\Mono\lib\mono\2.0\gmcs.exe"  @Temp/UnityTempFile-b0a7b727565cc0c498e9130d84f116bc
 MONO_PATH: C:\Program Files\Unity\Editor\Data\Mono\lib\mono\2.0
 MONO_CFG_DIR: C:\Program Files\Unity\Editor\Data\Mono\etc

but when I run it on Travis CI (the server is OS X), it produces a errors, and its using web player settings instead:

 -----Compiler Commandline Arguments:
 Filename: /Applications/Unity/Unity.app/Contents/Frameworks/Mono/bin/mono
 Arguments: '/Applications/Unity/Unity.app/Contents/Frameworks/Mono/lib/mono/unity_web/smcs.exe'  @Temp/UnityTempFile-66ade436a9eb04196ba8b0f2c4ca61e6
 MONO_CFG_DIR: /Applications/Unity/Unity.app/Contents/Frameworks/Mono/etc
 MONO_PATH: /Applications/Unity/Unity.app/Contents/Frameworks/Mono/lib/mono/unity_web

Whats going on? Why is the same build script producing drastically different results?