Stop testing libclang 3.9

Author: kulp

appveyor.yml

@@ -2,9 +2,6 @@ environment:
   RUST_BACKTRACE: 1
   RUST_CHANNEL: "%Configuration%"
   matrix:
-  - TARGET: gnu
-    LLVM_VERSION: 3.9.0-2
-    BINDGEN_FEATURES: testing_only_libclang_3_9
   - TARGET: gnu
     LLVM_VERSION: 4.0.0-1
     BINDGEN_FEATURES: testing_only_libclang_4
@@ -14,9 +11,6 @@ environment:
   - TARGET: gnu
     LLVM_VERSION: 9.0.0-1
     BINDGEN_FEATURES: testing_only_libclang_9
-  - TARGET: msvc
-    LLVM_VERSION: 3.9.0
-    BINDGEN_FEATURES: testing_only_libclang_3_9
   - TARGET: msvc
     LLVM_VERSION: 4.0.0
     BINDGEN_FEATURES: testing_only_libclang_4