build: require clang >= 3.6

3.5 fails with "error: debug information for auto is not yet supported"

Change-Id: I77ac4a064959b26796a4bf3f1dea7c3c08ea60a3
diff --git a/.travis.yml b/.travis.yml
index fb2ec6f..ef557b6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,8 +22,6 @@
     # Linux/clang
     # https://apt.llvm.org/
     - os: linux
-      env: COMPILER=clang++-3.5
-    - os: linux
       env: COMPILER=clang++-3.6
     - os: linux
       env: COMPILER=clang++-3.7 DISABLE_ASAN=yes
@@ -46,26 +44,24 @@
       osx_image: xcode7.3
       env: OSX_VERSION=10.11
     - os: osx
+      osx_image: xcode8
+      env: OSX_VERSION=10.11
+    - os: osx
       osx_image: xcode8.3
       env: OSX_VERSION=10.12
     - os: osx
-      osx_image: xcode9.3
+      osx_image: xcode9.2
+      env: OSX_VERSION=10.12
+    - os: osx
+      osx_image: xcode9.4
       env: OSX_VERSION=10.13
     - os: osx
-      osx_image: xcode9.3
+      osx_image: xcode9.4
       env: OSX_VERSION=10.13 USE_OPENSSL_1_1=yes
 
   allow_failures:
-    - env: COMPILER=g++-6
-    - env: COMPILER=g++-7
-    - env: COMPILER=g++-8
-    - env: COMPILER=clang++-3.5
     - env: COMPILER=clang++-3.6
     - env: COMPILER=clang++-3.7 DISABLE_ASAN=yes
-    - env: COMPILER=clang++-4.0
-    - env: COMPILER=clang++-5.0
-    - env: COMPILER=clang++-6.0
-    - env: COMPILER=clang++-7
     - env: OSX_VERSION=10.13 USE_OPENSSL_1_1=yes
 
   fast_finish: true