Bump javacpp.version in pom.xml to 1.5.4 to avoid build issue on Mac OS 11.5

[kkrugler]

Without this, trying to build the project results in:

ld: framework not found JavaVM
clang: error: linker command failed with exit code 1 (use -v to see invocation)

See https://stackoverflow.com/questions/64844595/javacpp-builds-failing-on-macos-after-xcode-12-2-update-due-to-missing-javavm-fr for details

(as a side note, I'd suggest updating the docs to specify mvn install vs. mvn package, so that the jar w/Mac OS binaries gets installed into the local Maven cache).