build: align minimum build dependencies with ndn-cxx

 * Recommend gcc >= 7.4.0
 * Require clang >= 4.0, or Xcode >= 9.0 on macOS
 * Silence an ABI-related diagnostic message from gcc on armv7
 * Update Travis CI job matrix

Refs: #5087, #5106
Change-Id: I71aef00147a7ad93b537904bb309745fed77f509
diff --git a/.travis.yml b/.travis.yml
index 54ce733..2e7caab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -38,6 +38,12 @@
     - os: osx
       osx_image: xcode11.3
       env: # default compiler
+    - os: osx
+      osx_image: xcode11.6
+      env: # default compiler
+    - os: osx
+      osx_image: xcode12
+      env: # default compiler
 
   allow_failures:
     - arch: s390x # bug 5098