CI: Add gcc8 to the test matrix

Signed-off-by: Larry Gritz <[email protected]>

.travis.yml

@@ -177,7 +177,7 @@ matrix:
               - libboost-wave1.55
         env: WHICHGCC=7 USE_CPP=11 LLVM_VERSION=7.0.0 BUILD_CMAKE=1
         if: branch =~ /(master|RB|travis)/ OR type = pull_request
-      - name: "Future: gcc8, C++14, LLVM 7, OIIO master, AVX/f16c"
+      - name: "Future: gcc8, C++14, LLVM 8, OIIO master, AVX/f16c"
         os: linux
         compiler: gcc
         addons:
@@ -192,7 +192,7 @@ matrix:
               - libboost-system1.55
               - libboost-thread1.55
               - libboost-wave1.55
-        env: WHICHGCC=8 USE_CPP=14 LLVM_VERSION=7.0.0 USE_SIMD=avx,f16c BUILD_CMAKE=1
+        env: WHICHGCC=8 USE_CPP=14 LLVM_VERSION=8.0.0 USE_SIMD=avx,f16c BUILD_CMAKE=1
       - name: "DEBUG build, LLVM 7, OIIO release"
         os: linux
         compiler: gcc