Appveyor: only run rocket tests on nightly + msvc

Rocket is only supported on nightly, and rocket depends on Ring, which
does not support the gcc toolchain on Windows.

appveyor.yml

@@ -5,23 +5,23 @@
 environment:
   matrix:
     # Stable channel
-    # - TARGET: i686-pc-windows-gnu
-    #   CHANNEL: stable
-    # - TARGET: i686-pc-windows-msvc
-    #   CHANNEL: stable
-    # - TARGET: x86_64-pc-windows-gnu
-    #   CHANNEL: stable
-    # - TARGET: x86_64-pc-windows-msvc
-    #   CHANNEL: stable
+    - TARGET: i686-pc-windows-gnu
+      CHANNEL: stable
+    - TARGET: i686-pc-windows-msvc
+      CHANNEL: stable
+    - TARGET: x86_64-pc-windows-gnu
+      CHANNEL: stable
+    - TARGET: x86_64-pc-windows-msvc
+      CHANNEL: stable
     # Beta channel
-    # - TARGET: i686-pc-windows-gnu
-    #   CHANNEL: beta
-    # - TARGET: i686-pc-windows-msvc
-    #   CHANNEL: beta
-    # - TARGET: x86_64-pc-windows-gnu
-    #   CHANNEL: beta
-    # - TARGET: x86_64-pc-windows-msvc
-    #   CHANNEL: beta
+    - TARGET: i686-pc-windows-gnu
+      CHANNEL: beta
+    - TARGET: i686-pc-windows-msvc
+      CHANNEL: beta
+    - TARGET: x86_64-pc-windows-gnu
+      CHANNEL: beta
+    - TARGET: x86_64-pc-windows-msvc
+      CHANNEL: beta
     # Nightly channel
     - TARGET: i686-pc-windows-gnu
       CHANNEL: nightly
@@ -49,10 +49,9 @@ install:
 build: false
 
 test_script:
-  - ECHO.%TARGET% | FIND "msvc">Nul && ( IF "%CHANNEL%"=="nightly" (cd juniper_rocket && cargo build --verbose && cargo test --verbose) )
   - cd juniper && cargo build --verbose && cargo test --verbose && cd ..
   - cd juniper_codegen && cargo build --verbose && cargo test --verbose && cd ..
   - cd juniper_iron && cargo build --verbose && cargo test --verbose && cd ..
   - cd juniper_tests && cargo build --verbose && cargo test --verbose && cd ..
-
+  - IF NOT %TARGET% == %TARGET:msvc=% ( IF %CHANNEL% == "nightly" ( cd juniper_rocket && cargo test --verbose && cargo build --verbose && cd .. ) )