travis: Allow CI config to make check-valgrind

Setting MAKE_CHECK_SUFFIX=-valgrind in the environment will cause Travis
to make check-valgrind.

.travis.yml

@@ -47,7 +47,7 @@ install:
     - ulimit -c unlimited; make -j3
 
 script:
-    - if [ x${COVERITY_SCAN_BRANCH} != x1 ]; then ulimit -c unlimited; make check; fi
+    - if [ x${COVERITY_SCAN_BRANCH} != x1 ]; then ulimit -c unlimited; make check${MAKE_CHECK_SUFFIX}; fi
 
 after_script:
     - if [ -n "$COVERAGE" ]; then ../tools/coveralls-tool -O $PWD -S ..; fi