travis: lower SWR requirement to GCC 4.8, aka std=c++11

With ealier commit we relaxed the requirement from C++14 to C++11.
Update the build script so that it

Cc: Tim Rowley <timothy.o.rowley@intel.com
Fixes: 0b80b02502 ("swr: relax c++ requirement from c++14 to c++11")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

.travis.yml

@@ -57,8 +57,8 @@ matrix:
         - MAKE_CHECK_COMMAND="true"
         - LLVM_VERSION=3.9
         - LLVM_CONFIG="llvm-config-${LLVM_VERSION}"
-        - OVERRIDE_CC="gcc-5"
+        - OVERRIDE_CC="gcc-4.8"
-        - OVERRIDE_CXX="g++-5"
+        - OVERRIDE_CXX="g++-4.8"
         - DRI_LOADERS="--disable-glx --disable-gbm --disable-egl"
         - DRI_DRIVERS=""
         - GALLIUM_ST="--enable-dri --disable-opencl --disable-xa --disable-nine --disable-xvmc --disable-vdpau --disable-va --disable-omx --disable-gallium-osmesa"
@@ -67,13 +67,11 @@ matrix:
       addons:
         apt:
           sources:
-            - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-3.9
           packages:
             # LLVM packaging is broken and misses these dependencies
             - libedit-dev
             # From sources above
-            - g++-5
             - llvm-3.9-dev
             # Common
             - xz-utils
@@ -250,19 +248,17 @@ matrix:
         - LLVM_CONFIG="llvm-config-${LLVM_VERSION}"
         # Keep it symmetrical to the make build. There's no actual SWR, yet.
         - SCONS_CHECK_COMMAND="true"
-        - OVERRIDE_CC="gcc-5"
+        - OVERRIDE_CC="gcc-4.8"
-        - OVERRIDE_CXX="g++-5"
+        - OVERRIDE_CXX="g++-4.8"
       addons:
         apt:
           sources:
-            - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-3.9
           packages:
             - scons
             # LLVM packaging is broken and misses these dependencies
             - libedit-dev
             # From sources above
-            - g++-5
             - llvm-3.9-dev
             # Common
             - xz-utils