Update build.sh

Smaller fix (never copy & paste...)

Author: leetal

.travis/build.sh

@@ -9,7 +9,7 @@ BUILD_SHARED=${BUILD_SHARED:-0}
 USE_XCODE=${USE_XCODE:-0}
 BUILD_CURL=${BUILD_CURL:-0}
 USE_STRICT_COMPILER_CHECKS=${USE_STRICT_COMPILER_CHECKS:-0}
-DEPLOYMENT_TARGET=${BUILD_CURL:-11.0}
+DEPLOYMENT_TARGET=${DEPLOYMENT_TARGET:-11.0}
 
 SHARED_EXT=""
 if [[ ${BUILD_SHARED} -eq 1 ]]; then