CI: introduce scripts/build-vtest.sh for installing VTest

we install VTest for all CI systems, let us unify instalation
diff --git a/.travis.yml b/.travis.yml
index 1aa415a..3083e30 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -38,9 +38,7 @@
     if: type == cron
 
 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"
+  - scripts/build-vtest.sh
 
 script:
   - make -j$(nproc) ERR=1 TARGET=linux-glibc CC=$CC DEBUG=-DDEBUG_STRICT=1 $FLAGS