coveralls upload should not fail the CI build

- swallow error code
Replace lalloc() with xmalloc()
No OOM error in make_filter_cmd()
Delete the lalloc() function
update CONTRIBUTING.md
Update CONTRIBUTING.md with current info

- Finish what @ashleyh started.
- Add section for Getting Started with Neovim.
- Add section for commit messages.
- Trim down on wording to keep it short.