Drop centos6 due to old compiler.

.gitlab-ci.yml

@@ -80,13 +80,6 @@ debian-oldoldstable:
         - ./configure --enable-gpl --enable-gpl3 && make && make install
             # there's no `check` or `test` `make` target
 
-centos-6:
-    image: centos:6
-    script:
-        - yum --assumeyes groupinstall "Development Tools"
-        - yum --assumeyes install yasm gavl-devel libsamplerate-devel libxml2-devel ladspa-devel jack-audio-connection-kit-devel sox-devel SDL-devel gtk2-devel qt-devel libexif-devel libtheora-devel libvorbis-devel libvdpau-devel libsoup-devel liboil-devel python-devel alsa-lib pulseaudio-libs-devel
-        - ./configure --enable-gpl --enable-gpl3 && make && make install
-
 centos-7:
     image: centos:7
     script: