build,meta: switch to gcc-4.9 on travis

The version of `clang` provided in the Travis linux image uses
libstdc++4.8 whice is below our minimal supported version.

Switching to `make test -j1` is to avoid races during the test cycle
causes by the main target being "unstable", that is it always builds
some files, and relinks the binary, which is used by the test procedure.

PR-URL: #23778
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

.travis.yml

@@ -1,6 +1,4 @@
 language: cpp
-compiler:
-  - clang
 sudo: false
 cache: ccache
 os: linux
@@ -15,12 +13,15 @@ matrix:
         # Lint the first commit in the PR.
         - \[ -z "$TRAVIS_COMMIT_RANGE" \] || (echo -e '\nLinting the commit message according to the guidelines at https://goo.gl/p2fr5Q\n' && git log $TRAVIS_COMMIT_RANGE --pretty=format:'%h' --no-merges | tail -1 | xargs npx -q core-validate-commit --no-validate-metadata)
     - name: "Test Suite"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.9
       install:
+        - export CC='ccache gcc-4.9' CXX='ccache g++-4.9' JOBS=2
         - ./configure
         - make -j2 V=
       script:
-        - make -j2 test
-      before_install:
-        - export CXX="ccache clang++ -Qunused-arguments"
-        - export CC="ccache clang -Qunused-arguments -Wno-unknown-warning-option"
-        - export JOBS=2
+        - PARALLEL_ARGS='--flaky-tests=skip' make -j1 test