add gradle wrapper directory to .gitignore
add gradle wrapper directory to .gitignore


Former-commit-id: 46492d3d7c58bf5e007b42fd53dcc96e14918948
kermitt2/grobid,kermitt2/grobid,kermitt2/grobid,kermitt2/grobid,kermitt2/grobid,kermitt2/grobid