updated osx to macOS 10.15.7, JDK to 14.0.2, and XCode to Xcode 12.2 (SheffieldML#904)

The CI  was broken. This commit fixes the CI. The root cause is reported in more detail in issue SheffieldML#905.

In short, the default macOS version (10.13, see the TravisCI docs) used in TravisCI isn't supported by brew which caused the brew install pandoc in the download_miniconda.sh pre-install script to hang and time out the build. It failed even on inert PRs (adding a line to README, e.g.). Now, with the updated macOS version (from 10.13 to 10.15), brew is supported and the brew install pandoc command succeeds and allows the remainder of the CI build and test sequence to succeed.

Author: ekalosak

.travis.yml

@@ -1,5 +1,7 @@
 sudo: false
 
+osx_image: xcode12.2
+
 os:
 - osx
 - linux