upgrade leveldown to v6 because build fails on apple M1

[il3ven]

While installing dynalite on an Apple M1 machine I get build error in the package leveldown. dynalite is currently using leveldown@^5.6.0 and upgrading to leveldown@^6.0.3 should prevent the build error.

[email protected] has added support for Mac M1. I don't know if the breaking change in v6 affects dynalite or not and I don't have enough experience to find out. Hence, I couldn't open a PR myself. 😞

For breaking changes in leveldown@^6.0.0 you can refer this.