Rocket chip generator build error: sbt proxy

Question

I am trying to download Rocket chip generator and build a Rocket core RTL using the generator. The guidance is relatively simple, as in the following link. I only need to type a several lines of commands. https://github.com/ucb-bar/rocket-chip

However, when I finally try to generate the verilog files by the command 'make verilog' after all configurations, I get stuck to the following error which pretty much seems to be related to sbt dependencies.

[info] Resolving com.typesafe.sbt#sbt-ghpages;0.5.3 ... [error] Server access Error: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target url=~ [error] Server access Error: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target url=~[error] Server access Error: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target url=https:~

......................

[error] (*:update) sbt.ResolveException: unresolved dependency: com.typesafe.sbt#sbt-ghpages;0.5.3: not found [error] unresolved dependency: com.typesafe.sbt#sbt-site;0.8.1: not found [error] unresolved dependency: com.eed3si9n#sbt-buildinfo;0.5.0: not found

Note that, I replaced the URLs to '~' mark since I was unable to write more than 2 links via this post, being a noob in stack overflow.

Indeed, I do not find the upper listed repo sites, not even on my web browser. Does anybody have any idea how to build the rocket chips?