Remove Travis-CI allow_failures entry for CentOS 7

It seems that upstream CentOS image update fixed the Python import failure.
diff --git a/.travis.yml b/.travis.yml
index 227393f..97440f3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -123,9 +123,6 @@
       BUILD_ENV="-e CC=gcc -e CXX=g++"
 
 matrix:
-  allow_failures:
-    - env: TEST_NAME="All (CentOS)" BUILD_CMD="../cmake.sh" DISTRO=centos
-
   include:
     # QA jobs for code analytics and metrics
     #