CI: Build VTest with clang

Current VTest master fails to build using gcc, see vtest/VTest#27.

This patch is to be reverted once VTest is fixed.
diff --git a/.travis.yml b/.travis.yml
index 1aa415a..37b667b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -40,7 +40,7 @@
 install:
   - git clone https://github.com/VTest/VTest.git ../vtest
   # Special flags due to: https://github.com/vtest/VTest/issues/12
-  - make -C ../vtest FLAGS="-O2 -s -Wall"
+  - make -C ../vtest FLAGS="-O2 -s -Wall" CC=clang
 
 script:
   - make -j$(nproc) ERR=1 TARGET=linux-glibc CC=$CC DEBUG=-DDEBUG_STRICT=1 $FLAGS