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
-*.a
-*.o
-*.dylib*
-*.so
-*.so.*
-*_test
-db_bench
-leveldbutil
+# Editors.
+*.sw*
+.vscode
+.DS_Store
+
+# Build directory.
+build/
+out/