Revert to Mojave for macOS Travis-ci builds

[skip appveyor]
Builds on Catalina are failing due to 50min max time limit.
[publish binaries]

.travis.yml

@@ -44,7 +44,7 @@ env:
 jobs:
   include:
   - os: osx
-    osx_image: xcode12.2
+    osx_image: xcode11.3
     compiler: clang
   - os: linux
     dist: bionic

INSTALL-MAC

@@ -1,6 +1,6 @@
 Update Note: to build GoldenCheetah v3.6 we are using the Homebrew Package
 Manager to install dependencies, including Qt and GSL, on Travis-ci
-macOS Catalina build environment.
+macOS Mojave build environment.
 You can check the travis/osx folder for the complete and updated build scripts,
 the minimum Qt version known to work is 5.13 with Qt WebEngine and Qt Charts.
 GSL - GNU Scientific Library is a mandatory dependency starting with v3.6

travis/osx/script.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -ev
-CC=clang CXX=clang++ /usr/local/opt/qt5/bin/qmake -makefile -recursive QMAKE_CXXFLAGS_WARN_ON+="-Wno-unused-private-field -Wno-c++11-narrowing -Wno-deprecated-declarations -Wno-deprecated-register -Wno-nullability-completeness -Wno-sign-compare" QMAKE_CFLAGS_WARN_ON+="-Wno-deprecated-declarations -Wno-sign-compare"
+CC=clang CXX=clang++ /usr/local/opt/qt5/bin/qmake -makefile -recursive QMAKE_CXXFLAGS_WARN_ON+="-Wno-unused-private-field -Wno-c++11-narrowing -Wno-deprecated-declarations -Wno-deprecated-register -Wno-nullability-completeness -Wno-sign-compare -Wno-inconsistent-missing-override" QMAKE_CFLAGS_WARN_ON+="-Wno-deprecated-declarations -Wno-sign-compare"
 CC=clang CXX=clang++ make qmake_all
 CC=clang CXX=clang++ make -j4 sub-qwt --silent
 CC=clang CXX=clang++ make -j4 sub-src --silent || CC=clang CXX=clang++ make sub-src