Get solr from Princeton mirror; do not check the checksum.

[eddierubeiz]

Use the Princeton mirror url to keep travis tests from being blocked by apache for too many solr downloads.

Ignore the checksum for now.

When the checksum can also be fetched from Princeton, which will happen when cbeer/solr_wrapper#119 is merged, we'll turn validate back on.

See also:
cbeer/solr_wrapper#122
#1153

Fixes #1166.

[jrochkind]

How the heck are we now passing CI without having merged this? I forget the state of this PR, should we still merge it?

[eddierubeiz]

We don't need this after all! We can come back to it as needed.

[jrochkind]

We can't quite remember why we thought we needed this -- we were having troubles with the travis build being able to download solr, involving apache rate-limiting, trying to use mirror url to get around that, and then apparent bugs or lack of feature in the solr_wrapper mirror_url feature.

We aren't currently having any trouble with the build though. Maybe just because apache isn't currently rate limiting us?

We can revisit this if the problem happens again and we are refreshed as to context.