add *.hprof to gitignore
add *.hprof to gitignore

Tests that cause OOMs and create hprof files should not count as the
repo being dirty.

palantir/atlasdb,EvilMcJerkface/atlasdb,palantir/atlasdb,EvilMcJerkface/atlasdb,EvilMcJerkface/atlasdb,palantir/atlasdb