Update .gitignore.

The version in the repository covers the Makefile build. The new version
is simpler and contains entries relevant to the CMake build.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212661504

.gitignore

@@ -1,9 +1,8 @@
-build_config.mk
+# Editors.
-*.a
+*.sw*
-*.o
+.vscode
-*.dylib*
+.DS_Store
-*.so
+
-*.so.*
+# Build directory.
-*_test
+build/
-db_bench
+out/
-leveldbutil