chore(travis): re-encrypt secure keys for leanprover/lean

Travis-ci[1] supports encryption key feature[2]. The caveat is that
plain-texts are encrypted and decrypted using repository's public and
private keys. That is, if we migrate our blessed repo to another one, we
need to re-encrypt and update our secure keys in .travis.yml file:

 - GH_TOKEN : to push to OSX/Windows repositories
              (Github token for account "soonhokong")

 - DROPBOX_KEY : to push compiled binaries to DROPBOX
                 (DROPBOX_KEY=access token)

 - REPO : encrypt "BLESSED". If travis-ci is running on another
          repository, REPO variable is not decrypted to "BLESSED" because it
          will use a different private key. Many actions (i.e. upload to
          Dropbox, trigger OSX/Windows Builds) are protected by this
          condition.

 - COVERALLS_REPO_TOKEN: to push to https://coveralls.io
                         (https://coveralls.io/r/leanprover/lean)

[1]: http://travis-ci.com
[2]: http://docs.travis-ci.com/user/encryption-keys/
chore(travis): use OSX10.8.5 + g++-4.8.1 configuration
tests(util/rb_tree): wrap tst6 with #if !defined
doc(README.md): use new coveralls.io link for the blessed repo