Add clang-7 to Travis

.travis.yml

@@ -6,8 +6,6 @@ language: cpp
 
 sudo: false
 
-python: "2.7"
-
 branches:
   only:
     - master
@@ -86,7 +84,7 @@ matrix:
 
     - os: linux
       compiler: g++-8
-      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=11,14,17
+      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=11,14,17,2a
       addons:
         apt:
           packages:
@@ -192,6 +190,17 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-6.0
 
+    - os: linux
+      compiler: clang++-7
+      env: TOOLSET=clang COMPILER=clang++-7 CXXSTD=11,14,17,2a
+      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=11,14,1z