[travis] add posix build check for gcc 7 (#2685)

.travis.yml

@@ -100,9 +100,6 @@ matrix:
             - llvm-toolchain-trusty-5.0
           packages:
             - clang-5.0
-    - env: BUILD_TARGET="posix" CC="gcc" CXX="g++"
-      os: linux
-      compiler: gcc
     - env: BUILD_TARGET="posix" CC="gcc-5" CXX="g++-5"
       os: linux
       compiler: gcc
@@ -123,6 +120,16 @@ matrix:
           packages:
             - gcc-6
             - g++-6
+    - env: BUILD_TARGET="posix" CC="gcc-7" CXX="g++-7"
+      os: linux
+      compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - gcc-7
+            - g++-7
     - env: BUILD_TARGET="posix-ncp-spi" VERBOSE=1
       os: linux
       compiler: gcc

src/ncp/ncp_spi.cpp

@@ -330,7 +330,8 @@ void NcpSpi::PrepareTxFrame(void)
     case kTxStateHandlingSendDone:
         mTxState = kTxStateIdle;
 
-        // Fall-through to next case to prepare the next frame (if any).
+        // Fall through
+        // to next case to prepare the next frame (if any).
 
     case kTxStateIdle:
         PrepareNextSpiSendFrame();