Stop testing libclang 4.0

Author: kulp

appveyor.yml

@@ -2,18 +2,12 @@ environment:
   RUST_BACKTRACE: 1
   RUST_CHANNEL: "%Configuration%"
   matrix:
-  - TARGET: gnu
-    LLVM_VERSION: 4.0.0-1
-    BINDGEN_FEATURES: testing_only_libclang_4
   - TARGET: gnu
     LLVM_VERSION: 5.0.0-1
     BINDGEN_FEATURES: testing_only_libclang_5
   - TARGET: gnu
     LLVM_VERSION: 9.0.0-1
     BINDGEN_FEATURES: testing_only_libclang_9
-  - TARGET: msvc
-    LLVM_VERSION: 4.0.0
-    BINDGEN_FEATURES: testing_only_libclang_4
   - TARGET: msvc
     LLVM_VERSION: 5.0.0
     BINDGEN_FEATURES: testing_only_libclang_5