Added gcc 8, clang 6.0 and 7 CI jobs.

.travis.yml

@@ -105,6 +105,16 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      compiler: g++-8
+      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=03,11,14,17
+      addons:
+        apt:
+          packages:
+            - g++-8
+          sources:
+            - ubuntu-toolchain-r-test
+
     - os: linux
       compiler: clang++
       env: TOOLSET=clang COMPILER=clang++ CXXSTD=03,11
@@ -205,6 +215,28 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-5.0
 
+    - os: linux
+      compiler: clang++-6.0
+      env: TOOLSET=clang COMPILER=clang++-6.0 CXXSTD=03,11,14,1z
+      addons:
+        apt:
+          packages:
+            - clang-6.0
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-trusty-6.0
+
+    - os: linux
+      compiler: clang++-7
+      env: TOOLSET=clang COMPILER=clang++-7 CXXSTD=03,11,14,17
+      addons:
+        apt:
+          packages:
+            - clang-7
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-trusty-7
+
     - os: linux
       compiler: clang++-libc++
       env: TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=03,11,14,1z