Travis CI: Fixes to support AVX2 code

-- Use trusty for SIMD builds.  Ubuntu 12.04 is still using NASM 2.09.x,
   which isn't new enough to support AVX2.
-- Add a special test for the SSE2 code path, since it is no longer the
   default.

.travis.yml

@@ -11,6 +11,7 @@ matrix:
       env: BUILD_OFFICIAL=1
       osx_image: xcode7.2
     - os: linux
+      dist: trusty
       compiler: clang
       env:
         CFLAGS="-O1 -g -fsanitize=address -fno-omit-frame-pointer"
@@ -24,6 +25,7 @@ matrix:
       compiler: gcc
       env: CONFIGURE_FLAGS="--with-12bit"
     - os: linux
+      dist: trusty
       compiler: gcc
       env: CONFIGURE_FLAGS="--with-jpeg8"
       addons:
@@ -73,6 +75,7 @@ script:
         make test FLOATTEST=32bit;
       else
         make test FLOATTEST=sse &&
+        JSIMD_FORCESSE2=1 make test FLOATTEST=sse &&
         JSIMD_FORCENONE=1 make test FLOATTEST=32bit;
       fi &&
       popd;