Problem: clang-tidy 6 has issues with comments

Solution: bump CI to version 7

.travis.yml

@@ -99,14 +99,15 @@ matrix:
   - env: BUILD_TYPE=android CURVE=tweetnacl
     os: linux
     dist: trusty
-  - env: BUILD_TYPE=cmake DO_CLANG_FORMAT_CHECK=1 CLANG_FORMAT=/usr/local/clang-5.0.0/bin/clang-format
+  - env: BUILD_TYPE=cmake DO_CLANG_FORMAT_CHECK=1 CLANG_FORMAT=/usr/lib/llvm-7/bin/clang-format
     os: linux
     addons:
-    apt:
-      sources:
-        - llvm-toolchain-trusty-5.0
-      packages:
-        - clang-5.0
+      apt:
+        sources:
+        - llvm-toolchain-trusty-7
+        - ubuntu-toolchain-r-test
+        packages:
+        - clang-format-7
   - env: BUILD_TYPE=default POLLER=poll
     os: linux
   - env: BUILD_TYPE=default POLLER=select