Fixes stale blockstack-portal references
Fixes stale blockstack-portal references

At some point, blockstack-portal was renamed to blockstack-browser, but
these README references stuck around. This commit updates the
documentation to avoid confusion or further drift.

Closes #792

blockstack/blockstack-browser,blockstack/blockstack-browser,blockstack/blockstack-browser,blockstack/blockstack-portal,blockstack/blockstack-browser,blockstack/blockstack-portal,blockstack/blockstack-browser,blockstack/blockstack-portal,blockstack/blockstack-browser,blockstack/blockstack-portal,blockstack/blockstack-portal