build: align minimum build dependencies with ndn-cxx

 * Recommend boost >= 1.65.1 and gcc >= 7.4.0
 * Require clang >= 4.0, or Xcode >= 9.0 on macOS
 * Silence an ABI-related diagnostic message from gcc on armv7

Refs: #5087, #5106
Change-Id: I88ef0e0f183736f19ac414a84baebb0e1073381b
diff --git a/.travis.yml b/.travis.yml
index 3d5ee08..8990ac7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -18,7 +18,10 @@
 jobs:
   include:
     # Linux
+    - env: COMPILER=g++-5
+    - env: COMPILER=g++-6
     - env: COMPILER=g++-8
+    - env: COMPILER=clang++-4.0
     - env: COMPILER=clang++-5.0
     - env: COMPILER=clang++-7
     - env: COMPILER=clang++-8
@@ -38,8 +41,14 @@
     - os: osx
       osx_image: xcode11.3
       env: # default compiler
+    - os: osx
+      osx_image: xcode11.4
+      env: # default compiler
 
   allow_failures:
+    - env: COMPILER=g++-5
+    - env: COMPILER=g++-6
+    - env: COMPILER=clang++-4.0
     - env: COMPILER=clang++-11
 
   fast_finish: true