travis: install scons from pip

The ubuntu version provided by Travis is a bit old, and does not detect
correctly some C functions.

Use a more modern version through scons.

Reviewed-by: Andres Gomez <agomez@igalia.com>

.travis.yml

@@ -400,7 +400,6 @@ matrix:
       addons:
         apt:
           packages:
-            - scons
             # Common
             - xz-utils
             - x11proto-xf86vidmode-dev
@@ -419,7 +418,6 @@ matrix:
       addons:
         apt:
           packages:
-            - scons
             # LLVM packaging is broken and misses these dependencies
             - libedit-dev
             - llvm-3.3-dev
@@ -445,7 +443,6 @@ matrix:
           sources:
             - llvm-toolchain-trusty-5.0
           packages:
-            - scons
             # LLVM packaging is broken and misses these dependencies
             - libedit-dev
             # From sources above
@@ -502,6 +499,11 @@ install:
       pip3 install --user "meson<0.45.0";
     fi
 
+  # Install a more modern scons from pip.
+  - if test "x$BUILD" = xscons; then
+      pip2 install --user "scons>=2.4";
+    fi
+
   # Since libdrm gets updated in configure.ac regularly, try to pick up the
   # latest version from there.
   - for line in `grep "^LIBDRM.*_REQUIRED=" configure.ac`; do