-DJSONCPP_USE_SECURE_MEMORY=1 for cmake

Add allocator.h to amalgamated header
Test JSONCPP_USE_SECURE_MEMORY in Travis
diff --git a/travis.sh b/travis.sh
index 2b25f47..a9811ec 100755
--- a/travis.sh
+++ b/travis.sh
@@ -19,6 +19,8 @@
 
 cmake -DJSONCPP_WITH_CMAKE_PACKAGE=$CMAKE_PKG -DBUILD_SHARED_LIBS=$SHARED_LIB -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_VERBOSE_MAKEFILE=$VERBOSE_MAKE .
 make
+cmake -DJSONCPP_WITH_CMAKE_PACKAGE=$CMAKE_PKG -DBUILD_SHARED_LIBS=$SHARED_LIB -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_VERBOSE_MAKEFILE=$VERBOSE_MAKE -DJSONCPP_USE_SECURE_MEMORY=1 .
+make
 
 # Python is not available in Travis for osx.
 #  https://github.com/travis-ci/travis-ci/issues/2320