Travis: Checkout a specific glslang version

This will help avoid test output mismatches due to glslang changes

.travis.yml

@@ -7,6 +7,10 @@ os:
 sudo: required
 dist: trusty
 
+# We check out glslang at a specific revision to avoid test output mismatches
+env:
+  - GLSLANG_REV=b56f4ac72c57f5c50f14ddb0bf1f78eaaef21c2b
+
 before_script:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install python3; fi
   - git clone https://github.com/KhronosGroup/glslang.git glslang
@@ -14,6 +18,7 @@ before_script:
   - git clone https://github.com/KhronosGroup/SPIRV-Headers.git SPIRV-Tools/external/spirv-headers
 
 script:
+  - git -C glslang checkout $GLSLANG_REV
   - cd glslang && cmake . && make -j2 && cd ..
   - cd SPIRV-Tools && cmake . && make -j2 && cd ..
   - make -j2