Run tests with more versions of libclang

First, update our existing 3.8.X and 3.9.X libclang testing versions to their
latest point releases. Second, add testing on the latest 3.7.X release and the
latest 4.0.X release.

Author: fitzgen

.travis.yml

@@ -14,8 +14,10 @@ rust:
   - stable
 
 env:
-  - CARGO_TARGET_DIR=/tmp/bindgen LLVM_VERSION=3.8 BINDGEN_FEATURES=testing_only_llvm_stable
-  - CARGO_TARGET_DIR=/tmp/bindgen LLVM_VERSION=3.9 BINDGEN_FEATURES=
+  - CARGO_TARGET_DIR=/tmp/bindgen LLVM_VERSION=3.7.1 BINDGEN_FEATURES=testing_only_llvm_stable
+  - CARGO_TARGET_DIR=/tmp/bindgen LLVM_VERSION=3.8.1 BINDGEN_FEATURES=testing_only_llvm_stable
+  - CARGO_TARGET_DIR=/tmp/bindgen LLVM_VERSION=3.9.1 BINDGEN_FEATURES=
+  - CARGO_TARGET_DIR=/tmp/bindgen LLVM_VERSION=4.0.0 BINDGEN_FEATURES=
 
 cache:
   directories: