Enable lots of plugins for better CI coverage.

Travis has changed the canonical name for this a while ago.

(Now really.  The last commit for this was empty.)

Workaround for Travis failures as described in
 https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296/18

Fixes strange C++11 compatibility problems during GTest installation:

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1249:1: error: unknown type name 'constexpr'
 constexpr bool TypedTestCaseIsDeprecated() { return true; }

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1249:11: error: expected unqualified-id
 constexpr bool TypedTestCaseIsDeprecated() { return true; }

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1254:1: error: unknown type name 'constexpr'
 constexpr bool RegisterTypedTestCase_P_IsDeprecated() { return true; }

pip is bugged on arm64 and s390x. Add --no-cache-dir to fix. For
consistency, it doesn't hurt to add everywhere.

The packages from the Travis image are good enough, and this speeds up
the Travis build.

Don't bother sending analytics data for a CI runner.  This adds
traffic but doesn't bring anybody any benefit.

Expand $PATH at evaluation and not at assignment, which fixes the
problem that /usr/lib/ccache was added to $PATH between the
MATRIX_EVAL assignment and its evaluation.

Fixes Travis failure with Meson 0.50:

 ERROR: Could not detect Ninja v1.5 or newer

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Apparently, Travis-CI will never upgrade its ancient Ubuntu Trusty,
so I have to get Boost from somewhere else.

Thanks to Martin Hierholzer for his PPA.

Also test the newest available GCC version.

After commit a72d1200, the Travis-CI
build fails, because we have been using GCC 5 there...  so let's
switch to GCC 6 to test the oldest possible GCC version.

The clang/libc++ version in the default xcode8 image is unable to
compile MPD due to incomplete C++14 support.

Prepare for Mac OS X support by omitting those Ubuntu-specific
commands.

.. and not update-alternatives, which requires "sudo".