Travis: Fix server build env in matrix

Another stray quote that broke Travis, not sure how it got there.

.travis.yml

@@ -36,7 +36,7 @@ matrix:
       os: osx
       osx_image: xcode9.3
       compiler: clang
-    - env: GODOT_TARGET=server TOOLS=no CACHE_NAME=${GODOT_TARGET}-clang"
+    - env: GODOT_TARGET=server TOOLS=no CACHE_NAME=${GODOT_TARGET}-clang
       os: linux
       compiler: clang