Enable ccache on Travis CI (#9951)
Enable ccache on Travis CI (#9951)

While working on https://github.com/greenplum-db/gpdb/pull/9937 ("Build
ORCA with C++14."), I realized we took out ccache from Travis CI. This
patch set brings it back with some additional TLC.

Highlights:

* Remove deprecated [1][2] key "sudo: false"

* Set Travis language to C++

  Travis propagates the environment variables CXX (in addition to CC,
  which is exported when the language is C) when the language is C++.
  This prepares us for a forthcoming change to enable ORCA build in
  Travis.

* Care is taken to ensure ccache is actually used on macOS.

  The default settings in Travis somehow neglected it
  (travis-ci/travis-build#655) so we compensate for it.

* Similar care is taken to ensure we use ccache with Clang.

  The version of ccache used in Travis is just slightly older than when
  automatic Clang symlink supported was added.

* Show ccache stats (hit rate and more) for each build

Reference:
[1] https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
[2] Job config linting warning from Travis
xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb