Alias readpartial to read to avoid method_missing

When Zlib::GzipReader reads from a TarEntry it tries #readpartial before
trying #read which results in a NoMethodError from method_missing.  Now
we save one method call as, for TarEntry, the two methods behave
identically.
Ignore dev dependencies when installing gems

There's no need to check these as the resolver has already determined
the correct gems to use.