Remove Qt support for macOS 10.11

.travis.yml

@@ -308,7 +308,6 @@ matrix:
     sudo: required
     script: bash src/mxe-build.sh windows64-qt
 
-
   # Build MintCoin-Qt
   # OS X 10.10 (Yosemite)
   #  
@@ -318,9 +317,13 @@ matrix:
 
   # Build MintCoin-Qt
   # OS X 10.11 (El Capitan)
-  - os: osx
-    osx_image: xcode7.3
-    script: brew install qrencode qt && /usr/local/opt/qt/bin/qmake && make -j2
+  #
+  #   Error: You are using macOS 10.11.
+  #   We (and Apple) do not provide support for this old version.
+  #   You will encounter build failures and other breakages.
+  #   Please create pull requests instead of asking for help on Homebrew's
+  #   GitHub, Discourse, Twitter or IRC. As you are running this old version,
+  #   you are responsible for resolving any issues you experience.
 
   # Build MintCoin-Qt
   # OS X 10.12 (Sierra)