Updated package to use prebuilt leveldb

Using leveldown-prebuilt enables use on Windows systems which do not have
a suitable compiler suite installed.
Updated package.json: renamed to level-prebuilt
Updated .travis.yml: Removed 0.8 & notifications
Updated README: Updated badge URLs
Merge pull request #1 from jhermsmeier/leveldown-prebuilt

Update package to use prebuilt leveldown