refactor travis CI test

to add a `make all` target with gcc-6.
Note : should fail at this stage, due to contrib/seekable_format
diff --git a/.travis.yml b/.travis.yml
index a9c1db5..4959e97 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,7 +10,7 @@
 matrix:
   include:
     # Ubuntu 14.04
-    - env: Cmd='make gcc6install && CC=gcc-6 make clean uasan-test-zstd'
+    - env: Cmd='make gcc6install && CC=gcc-6 make -j all && make clean && CC=gcc-6 make clean uasan-test-zstd'
     - env: Cmd='make gcc6install libc6install && CC=gcc-6 make clean uasan-test-zstd32'
     - env: Cmd='make gcc7install && CC=gcc-7 make clean uasan-test-zstd'
     - env: Cmd='make clang38install && CC=clang-3.8 make clean msan-test-zstd'
@@ -26,8 +26,7 @@
     - env: Cmd='make arminstall && make aarch64fuzz'
     - env: Cmd='make ppcinstall && make ppcfuzz'
     - env: Cmd='make ppcinstall && make ppc64fuzz'
-    - env: Cmd='make -j uasanregressiontest'
-    - env: Cmd='make -j msanregressiontest'
+    - env: Cmd='make -j uasanregressiontest && make clean && make -j msanregressiontest'
 
     - env: Cmd='make lz4install && make -C tests test-lz4'